Iodide of Bromine.—(See page 76.)

Experiments with Iodine.—Place a plate which has been exposed in the camera over the vapor of iodine for a very brief period, and it will present the appearance of the impression having been solarized.

b. Upon a Daguerreotype plate, from which an impression has been effaced by rubbing or otherwise, the picture may be made to reappear by merely coating it over with iodine.

c. Place in a vessel a little water, into which put the smallest possible quantity of free iodine and add a little starch, and the liquid will instantly assume a blue color. Advantage is taken of this fact in the laboratory to detect the presence of iodine in liquids. The starch should be dissolved in boiling water and allowed to cool. There are numerous other interesting experiments that can be performed by the aid of iodine, but it is unnecessary here to consume more space.

CHLORINE.

History.—The Swedish chemist, Scheele, in 1774, while examining the action of hydrochloric acid on peroxide of manganese, first noticed this element. He called it dephlogisticated muriatic acid. It was afterwards, by the French nomenclaturists, termed oxygenated muriatic acid, conceiving it to be a compound of oxygen and muriatic acid. This view of its notice was corrected by Sir H. Davy (in 1809), who gave it the present name. In 1840-41, this gas vas employed for accelerating the operation of light upon the iodized Daguerreotype plate. John Goddard, Wolcott & Johnson, Claudet, Draper, Morse and others, were among the first made acquainted with its use. Count Rumford, Ritter, Scheele, Seebert and others, experimented with chlorine in regard to its effect when exposed to the action of light in combination with silver. In 1845, M. Edward Becquerel announced that he had "been successful in obtaining, by the agency of solar radiations, distinct impressions, of the colors of nature."

On the 4th of March, 1851, Neipce, St. Victor, a former partner of DAGUERRE, announced that he had produced "all the colors by using a bath of bichloride of copper, and that a similar phenomenon occurs with all salts of copper, mixed with chlorine."

Preparation.—This is easily accomplished by putting about two parts of hydrochloric (muriatic) acid on one of powdered black oxide of manganese, and heating it gradually in a flask or retort, to which may be adapted a bent glass tube. A yellowish-green gas is disengaged, which being conducted through the glass tube to the bottom of a bottle, can readily be collected, being much heavier than the air, displaces it completely and the bottle is filled (which can be seen by the green color); a greased stopper is tightly fitted to it, and another bottle may be substituted.

In all experiments with chlorine, care should be taken not to inhale the gas!

Properties.—Chlorine is a greenish-yellow gas (whence its name, from chloros, green), with a powerful and suffocating odor, and is wholly irrespirable. Even when much diluted with air, it produces the most annoying irritation of the throat, with stricture of the chest and a severe cough, which continues for hours, with the discharge of much thick mucus. The attempt to breathe the undiluted gas would be fatal; yet, in a very small quantity, and dissolved in water, it is used with benefit by patients suffering under pulmonary consumption.

Under a pressure of about four atmospheres, it becomes a limpid fluid of a fine yellow color, which does not freeze at zero, and is not a conductor of electricity. It immediately returns to the gaseous state with effervescence on removing the pressure.

Water recently boiled will absorb, if cold, about twice its bulk of chlorine gas, acquiring its color and characteristic properties. The moist gas, exposed to a cold of 32 deg., yields beautiful yellow crystals, which are a definite compound of one equivalent of chlorine and ten of water. If these crystals are hermetically sealed up in a glass tube, they will, on melting, exert such a pressure as to liquefy a portion of the gas, which is distinctly seen as a yellow fluid, not miscible with the water which is present. Chlorine is one of the heaviest of the gases, its density being 2.47, and 100 cubic inches weighing 76.5 grains.

Chlorine Water.—This combination, which is used in conducting M. Neipce's process, can be readily prepared by conducting the gas into a bottle containing distilled water. One part water dissolves two parts of chlorine.

Chlorides.—The metallic chlorides are nearly all soluble in water; that of silver and protochloride of mercury being the only exceptions. A metallic chloride, treated with oil of vitriol, disengages chlorohydric acid. Heated with a mixture of peroxide of manganese and sulphuric acid, chlorine is given off, which is easily recognized by its odor and other physical properties.

The chlorides dissolve in water; give with nitrate of silver, a white precipitate, even in highly diluted solutions, becoming violet colored and finally black when exposed to the light. The rapidity of the change of color is proportioned to the intensity of the light. It is insoluble in nitric acid, but readily soluble in ammonia; it fuses without decomposition, forming, when cold, a tough, horny mass, and is reduced by hydrogen and by fusion with carbonate of soda, or with resin.

Chloride of Bromine. (See page 74.)

Chloride of Iodine. (See page 85.)

Chloride of potassium.—or (Muriate of Potassa).—Dissolve half an ounce of carbonate of potassa in water, and neutralize with muriatic acid. Upon concentrating the solutions, cubic crystals will be obtained, having a taste similar to common salt. They consist of potassium and chloride, and when dissolved in water they may be regarded as muriate of potassa.

Chloride of Lime.—Mix half an ounce of slacked lime (hydrate of lime) with six ounces of water, and conduct into this milk of lime, with frequent agitation, as much chlorine gas as will evolve from two ounces of muriatic acid and half an ounce of black oxide of manganese. The liquid clarifies by standing; may be regarded as a solution of chloride of lime, and must be protected from the air and light. It may also be made without putting in the water with the hydrate of lime, by merely passing the chlorine into the hydrate of lime. This last is by some used in preparations for accelerating the operation of taking Daguerreotypes, but when used for this purpose it is in small quantities.

Chloride of Calcium.—To one part of water add two parts of muriatic acid, and add pieces of common chalk until effervescence ceases; then filter through cotton cloth and evaporate it by placing it in all earthen or porcelain dish, over a slow fire, to the consistency of a syrup. When cooling, large prismatic crystals of chloride of calcium are formed. These must be quickly dried by pressing between folds of blotting paper and kept carefully excluded from the air, as it readily attracts hydrogen. For most daguerreotype purposes, the syrup may be at once evaporated to dryness. This is frequently placed in the iodine coating box for the purpose of keeping the atmosphere dry. It is so easily made that every operator can provide himself with it in a short time, and at little expense.

Chloride of Gold.—Is prepared by dissolving gold in aqua regia, a composition of one part of nitric to two parts of muriatic acid. Gold foil is the best for our purposes; coin, however, answers, in most cases, for the daguerreotype operator, as the alloy, being so slight is not noticed in the gilding process. When the latter is used, it will facilitate the operation to beat it out, forming a thin sheet, and then cutting in small strips. Where purity is required, foil is better. The gold is placed in three or four times its own weight of the above acids. For this purpose, an evaporating dish is best (a common saucer will do); a moderate heat may be applied to favor the action. The mixture should be stirred often with a glass rod; care should be observed not to apply too much heat, for at a temperature of about 300 deg. the chlorine would be expelled and leave a metallic precipitate, which would require re-dissolving. Acid may at any time be added if necessary to dissolve the gold, but it is advisable to add as little excess as possible, as it would require more time to evaporate. After all the gold has dissolved, and the liquid assumes a deep red color, the solution should be allowed to cool, being stirred nearly all the time. This salt is of a reddish-brown color. It is rarely we find in our market good chloride of gold, as common, salt is used for the bulk; and when the bottles are labelled "15 grains," "20 grains," nine-tenths do not in reality contain exceeding five grains of chloride of gold. The salt is mixed with the above solution when it is cooling, and gives bright yellow crystals, which some of our uninformed operators conceive to be the best quality.

Chloride of Silver.—(Oxide of Silver.)—Take any quantity of silver coin or other silver, roll or hammer it thin; cut in small pieces. This in order to save time. Put the silver in a glass or earthen vessel (Florence flask is best); pour in nitric acid and water, about three parts of the former to one of the latter. The operation of cutting up the silver may be facilitated by applying a gentle heat. This blue solution consists of oxide of silver and oxide of copper, both combined with nitric acid. Should the operator wish a pure solution of silver, which, however, is not always used, he may obtain it in the following manner:

To separate the two metals contained in the above solution from each other, put some bright copper coins into the solution and set it aside in a warm place for three or four days, occasionally giving it a circular motion. The separated laminae are pure silver, which is to be digested with ammonia until it ceases to be colored blue. The silver, after being washed and dried, is again dissolved in nitric acid, and the liquid, diluted with water, is kept as solution, of silver.

Either of the above solutions (the one of oxide of silver and copper, and the pure silver solution) may be prepared for use by putting them in a bottle, with a quantity of water, and adding common fine salt, you obtain a white curdy precipitate of chloride of silver. No matter how much salt is used, provided enough be added to throw down all the chloride of silver. This solution should be well agitated and then allowed to stand for a few minutes; thus the white precipitate is in the bottom of the bottle. When the water has become clear, pour it off with care, leaving the sediment behind, then add a fresh quantity of clean water, shake, let settle, and pour off as before. Repeat the same for several times, and the excess of salt will disappear, leaving the white precipitate, which may be drained of the water and dried in the dark, and kept free from light and air.

CYANIDE OF POTASSIUM.

Cyanide of Potassium.—This important article is worthy the undivided attention of every Daguerreotypist. I here give Mr. Smee's process for its preparation. This is from that author's work entitled, "Electro Metallurgy," American edition:

"The cyanide of potassium, so often alluded to while treating of the metallo-cyanides, may be formed in several ways. It may be obtained by heating to a dull redness the yellow ferrocyanate of potash, in a covered vessel, filtering and rapidly evaporating it. The objection to this method, however, is that without great care the whole of the ferrocyanate is not decomposed, a circumstance which much reduces its value for electro-metallurgy. By boiling, however, the ignited residue with spirits of wine this difficulty is said to be overcome, as the ferrocyanate is absolutely insoluble in that menstruum, while the cyanuret, at that heat, freely dissolves, and is as easily re-deposited on cooling.

"There is, however, a much better process by which this salt may be formed, viz. by simply transmitting hydrocyanic acid through potassium. Although the modes of making this acid are very numerous, there is but one which is likely to be employed on a very large scale, and that is its formation from the yellow ferrocyanate by means of sulphuric acid. This process is performed as follows: any given weight of the yellow salt is taken and dissolved in about five times its weight of water; this is placed in a retort, or some such analogous vessel, to which is then added a quantity of strong sulphuric acid, twice the weight of the salt, and diluted with three or four times its quantity of water. A pipe is carried from the neck of the retort to the receiving bottle, which should be kept as cool as possible.

"For small operations, those invaluable vessels, Florence flasks, answer well: a bent tube being connected at one end to its month, the other passing into the second vessel; heat should be cautiously applied by means of an Argand lamp, a little vessel of sand being placed under the flask, which helps the acid to decompose the salt. Prussic acid is then generated and passes through the tube to the recipient vessel, which is to be charged with liquor potassae.

"When the potash is saturated, the operation is completed. The Germans recommend a strong, alcoholic solution of potassa to be used in the second vessel, for in this case, the hydrocyanic or prussic acid combines with the potassa, forming a hydrocyanate of potassa, or, the water being abstracted, the cyanuret of potassium, which spontaneously precipitates, on the saturation of the fluid, the cyanuret, being insoluble in strong alcohol. The ferrocyanate of potash may be considered as containing three equivalents of hydrocyanic acid, two of potash and one of iron; but, unfortunately, we can only obtain half the acid from the salt, owing to the formation of a compound during its decomposition which resists the action of the acid. The decomposition of this salt taking 2 equivalents or 426 grains (to avoid fractions) would afford 3 equivalents or 81 grains of hydrocyanic, or prussic acid, capable of forming 198 grains of cyanuret of potassium, while in the retort there would remain 384 grains or 3 equivalents of bisulphate of potash, and 1 equivalent or 174 grains of a peculiar compound, said to contain 3 equivalents of cyanogen, 1 of potassium, and one of iron (Pereira). It is manifest that, but for this later compound, we might double the quantity of hydrocyanic acid from the yellow salt."

The decomposition just described is the one usually received; but too much reliance must not be placed on its accuracy, for the analysis of the several compounds is too difficult for the results to be fully admitted. The residue left in the retort speedily turns to one of the blues, identical with, or allied to, Prussian blue. This is at best a disagreeable process to conduct, for the hydrocyanic acid formed adheres so strongly to the glass, that, instead of being freely given off, bubbles are evolved suddenly with such explosive violence as occasionally to crack the vessel. This may be remedied as far as possible by the insertion of plenty of waste pieces of platinum—if platinized, so much the better, as that facilitates the escape of the gas. The heat should be applied to every part of the vessel, and the flame should not be allowed to play upon one single part alone. Large commercial operations are performed in green glass or stone-ware retorts.

"Now for one word of advice to the tyro: Remember that you are working with prussic acid; therefore, never conduct the process in a room, the fumes being quite as poisonous as the solution of the acid itself; moreover, have always a bottle of ammonia or chlorine by your side, that should you have chanced to inhale more than is pleasant, it will be instantly at hand to counteract any bad effects. It is stated by Pereira, that a little sulphuric acid or hydroferrocyanic acid passes to the outer vessel, but probably the amount would be of no consequence for electro-metallurgy, otherwise, it might be as well to use a Woulfe's apparatus, and discard the salt formed in the first vessel. To the large manufacturer it may be worth considering whether some other metallo-cyanuret, formed in a similar manner to the ferrocyanuret, might not be more advantageously employed, because the residue of the process last described contains a large quantity of cyanogen which the acid is unable to set free.

"There are other modes of procuring prussic acid, besides the one which has been so tediously described; but these are found to be more expensive. The only one which I shall now notice is the process by which it is obtained from bicyanide of mercury. The bicyanide of mercury itself is formed when peroxide of mercury is digested with Prussian blue, the peroxide of mercury abstracting the whole of the cyanogen from the blue, and leaving the oxides of iron at the bottom of the vessel. The solution may be evaporated to dryness, and one part of the salt dissolved in six of water; one part of muriatic acid, sp. gr. 1.15, is then added, and the solution distilled, when the whole of the hydrocyanic acid passes over, and by being conducted into a solution of potassa, as in the former process, forms cyanuret of potassium. This process, though easier than the first described, is rather given as a resource under peculiar circumstances than as one to be adopted by the large manufacturer. The expense is the only objection, but in a small quantity this cannot be a consideration.

"In giving this very rough outline of the general mode of forming salts, the minutiae necessary for chemical work have altogether been avoided, and those parts alone are entered upon which are more immediately necessary for the electro metallurgist to know and practice for himself. This will account for the long description of the cyanuret of potassium, while the preparation of the equally important and even more used acids, the sulphuric, muriatic, etc., commonly found in commerce, are altogether neglected.

"In using solutions of cyanide of potassium, the workman should not immerse his arms into them, otherwise it occasionally happens that the solution produces very troublesome eruptions over the skin."

HYPOSULPHITE OF SODA.

Hyposulphite of Soda.—This salt forms one of the important chemicals for the Daguerreotype operator. Its application to this art is of an interesting nature. It is used to dissolve the sensitive salt of silver which remains unchanged during the exposure in the camera. It has the property of readily dissolving the chloride, bromide and iodide of silver. It should be pure and free from sulphuret of sodium; should this last be present, it will cause brown spots of sulphurated silver upon the Daguerreotype impression. This annoyance is a great source of complaint from many operators, and ever will be, so long as it is prepared by men who have no reputation to lose, and whose eyes are blinded by the "Almighty Dollar."

A good article may be prepared as follows:

"Mix one pound of finely pulverized carbonate of soda with ten ounces of flowers of sulphur, and heat the mixture slowly in a porcelain dish till the sulphur melts. Stir the fused mass, so as to expose all its parts freely to the atmosphere, whereby it passes from the state of a sulphuret, by the absorption of atmospheric oxygen, into that of a sulphite, with the phenomenon of very slight incandescence. Dissolve in water, filter the solution, and boil it immediately along with flowers of sulphur. The filtered concentrated saline liquid will afford, on cooling, a large quantity of pure and beautiful crystals of hyposulphite of soda."

Hyposulphite of Gold.—This compound salt is by a few considered preferable to the chloride of gold, but our experience has induced us to use the latter, believing we are enabled to produce a more brilliant and warm-toned impression with it. When the hyposulphite of gold is used in gilding, it requires less heat and a longer application, as there is some danger of producing a glossy scum over some parts of the surface of the plate. I prepare this salt as follows:

Dissolve one part chloride of gold and four parts hyposulphite of soda in equal quantities of distilled water: pour the gold into the hyposulphite solution, in the same manner as in mixing the gilding solution; let it stand until it becomes limpid; filter and evaporate to dryness. Re-dissolve and add a few grains of burnt alum.

After standing a few hours, filter and evaporate again. If not sufficiently pure, repeat the crystallization until it is so. For gilding, dissolve in water and use in the same manner as the common gilding solution.

N.B.—The four following mixtures were employed in Neipce's process in his earliest experiments:

Aqueous Solution of Bichloride of Mercury.—Eight grains of bichloride of mercury in 10,000 grains of distilled water.

Solution of Cyanide of Mercury.—A flask of distilled water is saturated with cyanide of mercury, and a certain quantity is decanted, which is diluted with an equal quantity of distilled water.

Acidulated White Oil of Petroleum.—This oil is acidulated by mixing with it one tenth of pure nitric acid, leaving it for at least 48 hours, occasionally agitating the flask. The oil, which is acidulated, and which then powerfully reddens litmus paper, is decanted. It is also a little colored, but remains very limpid.

Solution of Chloride of Gold and Platinum.—In order not to multiply the solutions, take the ordinary chloride of gold, used for fixing the impressions, and which is composed of 1 gramme of chloride of gold and 50 grains of hyposulphate of soda, to a quart of distilled water.

With respect to chloride of platinum, 4 grains must be dissolved in 3 quarts of distilled water; these two solutions are mixed in equal quantities.

Acids.—I shall not go into the preparations of the various acids employed in the Daguerreotype. This would be useless to the operator, as there are few, if any, that it would be advisable to prepare. It is only necessary for the experimenter to be made acquainted with their properties, and this in order to prevent any haphazard experiments, which are too common among operators. Any person who may be desirous to try an experiment, should first study the agents he wishes to employ. By so doing much time and money will be saved; while the searcher after new discoveries would rarely become vexed on account of his own ignorance, or be obliged to avail himself of the experience of others in any department of science.

Nitric Acid—Exists in combination with the bases, potash, soda, lime, magnesia, in both the mineral and vegetable kingdoms, and is never found insoluble. It has the same constituents as common air, but in different proportions. The strongest nitric acid contains in every pound, two and a quarter ounces of water. Pure nitric acid is colorless, with a specific gravity of 1.5, and boiling at 248 deg.. It is a most powerful oxidizing agent, and is decomposed with more or less rapidity, by almost all the metals, to which it yields a portion of its oxygen.

The nitric acid of commerce, is generally the article used by the Daguerreotypist. This usually contains some chlorine and sulphuric acid. It is obtained by the distillation of saltpetre with sulphuric acid. It is employed in the Daguerreotype process for dissolving silver, preparing chloride or oxide, nitrate of silver, [the former used in galvanizing,] and in combination with muriatic acid for preparing chloride of gold, used in gilding. It is also used by some for preparing the plate.

Acidulated Solution.—This solution is used for cleaning the surface of the Daguerreotype plate. It has the property of softening the silver, and bringing it to a state in which it is very susceptible of being either oxidized or iodized, hence it contributes to increase the sensibility of the plate. The proportions are to one drop of acid add from 15 to 20 drops of water, or make the solution about like sharp vinegar to the taste.

Nitro-Muriatic Acid.—Aqua Regia is a compound menstruum invented by the alchemists for dissolving gold. It is composed of colorless nitric acid (aqua-fortis) and ordinary muriatic acid; the mixture is yellow, and acquires the power of dissolving gold and platinum. These materials are not properly oxidized; it nearly causes their combination with chlorine, which is in the Muriatic acid.

Hydrochloric Acid (Muriatic Acid).—This acid forms a valuable addition to the chemicals employed by the practical Daguerreotypist. This acid is formed by acting upon common salt (which is chloride of sodium) by concentrated sulphuric acid. The water of the acid is decomposed, and its hydrogen combines with the chloride of the salt to form muriatic acid, and this unites with the sulphuric acid to form sulphate of soda; 60 parts of common salt and 49 parts of concentrated sulphuric acid, afford, by this mutual action, 37 parts of muriatic acid and 72 parts of sulphate of soda. The muriatic acid of commerce has usually a yellowish tinge, but when chemically pure it is colorless. The former is commonly contaminated with sulphurous acid, sulphuric acid, chlorine, iron, and sometimes with arsenic.

Muriatic acid, from the fact of the presence of the chlorine, is used in the Daguerreotype process for dissolving gold, and in combination with various accelerators. Its presence can be detected by ammonia. A strip of paper dipped in this and waved to and fro will emit a thick white smoke if the acid vapor be in the atmosphere. The ammonia neutralizes the acid fumes. By reversing the experiment we can determine whether vapor of ammonia be in the air, and also deprive these suffocating and dangerous gases of their injurious properties, and remove them from the air. Every Daguerreotype operator should be furnished with, at least, a six ounce bottle of aqua ammonia. Its operation is very nearly the same on bromine and iodine vapor.

Hydrofluoric Acid (Fluorohydric Acid).—This acid is used to form some of the most volatile and sensitive compounds employed in the Daguerreotype. It is one of the most dangerous bodies to experiment with: it is volatile and corrosive, giving off dense white fumes in the air. It combines with water with great heat. At 32 deg. it condenses into a colorless fluid, with a density 1.069. It is obtained from decomposition of fluorspar by strong sulphuric acid. It readily dissolves the silica in glass, and consequently cannot be kept in a vessel of that material. It is prepared and kept in lead. It is employed in accelerators on account of its fluorine.

One small drop on the tongue of a dog causes death. The operator who wishes to use it should pour some of the liquid for which he intends it into a graduate, or other vessel, and then add the desired quantity of acid. If by accident any of the spray should fall upon the skin, it should at once be copiously drenched with water.

Sulphuric Acid.—There are two sorts of this acid: one is an oily, fuming liquid; this is made in Nordhausen, in Saxony, and is commonly called "Nordhausen sulphuric acid," or oil of vitriol. The other which is the kind used in connection with the Daguerreotype, is common sulphuric acid. It is somewhat thinner, and when undiluted is not fuming. This acid may be obtained in a solid and dry state, called anhydrous sulphuric acid.

The common sulphuric acid is made by burning sulphur, which forms sulphurous acid. To convert this into sulphuric acid and gain more oxygen, nitric acid, which is rich in that body, is added. It forms a limpid, colorless fluid, of a specific gravity of 1.8. It boils at 620 deg.; it freezes at 15 deg. It is acrid and caustic, and intensely acid in all its characters, even when largely diluted.

Its attraction for basis is such that it separates or expels all other acids, more or less perfectly, from their combinations. Its affinity for water is such that it rapidly absorbs it from the atmosphere, and when mixed with water much heat is evolved. It acts energetically upon animal and vegetable substances, and is a poisonous, dangerous substance to get on the skin. It is a powerful oxidizing agent; hence its use in the galvanic battery, for which purpose it is mostly used by the Daguerreotypist. The fumes of this being so much more offensive than nitric acid, the latter is sometimes used. It is also employed in some of the more sensitive accelerators.

ACCELERATING SUBSTANCES.

Remarks on the Accelerating substances Used in the Daguerreotype.—I have now arrived at a point in this work, where the eye of the Daguerreotype public will intently search for something new. This search will prove in vain, at least so far as regards those who have enjoyed and embraced the opportunities for studying the principles of our art. Every experienced operator has in a degree become familiar with the mechanical uses of all the agents employed, while I fear but few understand the properties, and laws governing those properties, which are so indispensable to produce an image impressed upon the silver surface.

There are three substances which form the bases for producing a Daguerreotype; silver, iodine and bromine. Each forms a separate body which is indispensable to the operators success as the art is now practiced in America. With these three, compounds of great variety are formed.

The silver surface is first thoroughly cleaned and freed from all organic matter, then exposed to vapor of iodine, producing an iodide of silver. The plate upon which is this salt, is again exposed to the vapor of bromine, forming a bromo-iodide of silver, a salt also.

As most of the various accelerators are compounds of bromine, with either chlorine or fluorine combination, they partake somewhat of the nature of these latter, giving results which can be detected by the experienced operator. Thus muriatic acid is added for its chlorine, which can generally be detected by the impression produced, being of a light, soft, mellow tone, and in most cases presenting a brilliant black to that colored drapery. Those who wish to experiment with agents for accelerating substances, should first study to well understand their peculiar nature and properties; as well, also, to endeavor to find out what will be the probable changes they undergo in combination as an accelerator. This should be done before making the experiments. From the foregoing it will be seen that numerous compounds are formed from the same basis, and, consequently, it would be a waste of time and a useless appropriation to devote more of our space than is necessary to give the principal and most reliable combination.

In America, the words "Quick" and "Quick Stuff," are more generally used for and instead of the more proper names, "Sensitives," or "Accelerators," etc. As it has by use become common, I frequently use it in this work.

Liquid Accelerator, No. 1.—This mixture was used by me in 1849, and is given as it appeared in my "System of Photography," published at the above date:

Take pure rain or distilled water, one quart, filter through paper into a ground stopper bottle, and add, for warm weather, one and a half ounce chloride of iodine; or for cold, one ounce; then add one ounce bromine, and shake well. Now with care not to allow the vapor to escape, add drop by drop, thirty drops of aqua ammonia, shaking well at each drop. Care must be taken not to add more at a time, as it evokes too much heat. This mixed, in equal proportions with John Roach's quick, forms an excellent chemical combination. For this purpose, take one and a half ounce of each, to which add ten ounces water, for warm weather, or from six to seven for cold. Pour the whole into a large box, and it will work from two to four months. I am now using (1849) one charged as above which has been in constant use for three months, and works uniformly well. The above is right for half or full size boxes, but half of it would be sufficient for a quarter size box.

Coat to the first shade of rose over iodine, change to a deep rosy red over quick, and black about one tenth the first.

I would not now recommend the addition of "John Roach's quick," as I believe equally good results can be produced without it. This liquid is now used by many, and is very good for taking views.

Lime Water Quick.—This mixture is more used at present than all the other liquids ever introduced. It produced the most uniform results, giving the fine soft tone so characteristic in pictures produces from accelerators containing chlorine. To one quart of lime water (this can be had of any druggist) add one and a half ounce of pulverized alum. This should be shook at intervals for twenty—four hours; then add one ounce of chloride of iodine and three fourths ounce of bromine.

Lime Water.—This is easily prepared by putting lime into water, say a piece of quick-lime about the size of an egg into one quart of water. This should be shook occasionally for two or three days and allowed to settle, when the water can be poured off and used.

Use.—To one part of quick add six parts of water; coat to a light yellow over the iodine, to a rose color over the quick, and recoat about one tenth. The above coating may be increased or diminished, it matters not, so that there is not too much, and the proper proportions are preserved. Some add to the above a small quantity of magnesia, say about a teaspoonful to the quart of liquid.

Liquid Accelerator, No. 2.—The following was for a long time used by one of the first houses in the United States, and probably was one of the first liquids ever used. It produces a fine-toned picture, but is not considered as sure as the lime water quick:

Take rain water one quart, add pulverized alum until it is a little sour to the taste, and a small piece, say one half inch square, of magnesia. Filter through paper, and add chloride of iodine one half ounce, bromine sufficient to take it up, which is a little less than half an ounce.

Charge with one of quick to six of water; coat over iodine to a soft yellow, nearly, but not quite, bordering on a rose; over quick to a dark purple, or steel, and back one sixth to one tenth.

Wolcott's American Mixture.—Van Loan Quick.—This mixture was first formed and used by T. Wolcott & Johnson and gained great celebrity for its productions. I have now a bottle hermetically sealed that contains about a half ounce of this mixture prepared in 1841 by John Johnson, now a resident of this city, and the former partner of Mr. Wolcott. The preparation of this mixture, as furnished by Mr. Johnson himself, is given as follows:

"One part of bromine, eight parts of nitric acid, sixteen parts of muriatic acid, water one hundred parts. This mixture should be allowed to stand for several days; it improves by age.

"Use.—A few drops say, 6 to 12, of this mixture, should be put into about 6 or 8 ounces of water; it will require frequent replenishing by the addition of a few more drops. The plate should be coated over the dry iodine to a red just bordering on a slate, and then exposed to the mixture only sufficiently long to change the color. If this is not done in less than six seconds it is not strong enough. Re-coat over the iodine full one fourth as long as first coating."

This exceedingly volatile compound is difficult to control from its instability; it is but little used. The impressions successfully produced by this mixture are very brilliant, and possess a pleasing peculiarity.

DRY SENSITIVES.

Hydrate of Lime.—The operation by which water is combined with lime is called slaking. Take a piece of quick lime, common lime used in mortar, and immerse it in warm water for about fifteen seconds; then place it in an iron or tin vessel. It will soon begin to swell, evolving a great deal of heat and emitting steam, and soon falls into a fine powder, hydrate of lime. This should be well stirred and allowed to cool, and then bottled in order to prevent it from giving off the hydrate and recovering the carbonic acid from the atmosphere. The last is detrimental to its use with bromine, and is one cause of the complaint that "it will not take bromine." The hydrate of lime should, not be dried over a heat, as has been supposed by many, for in that case the hydrogen is expelled and it returns to a carbonate. It is advisable to cool it in a damp place like a ground cellar. Much of the lime in our market will not, except it be quite damp, combine with the bromine. This is owing to impurities. Nothing is equal to oyster-shell lime, which I use altogether.

Bromide of Lime.—In preparing large quantities of this, we adopt the following method: Fill a four-quart bottle about two-thirds full of hydrate of lime; pour into this about one or two ounces of bromine; then shake well, add more of the bromine, shake well and let it stand for a few hours, adding sufficient bromine to give it a fine red color. It is better when kept in the large bottles, as it forms a more perfect combination: in other words it improves by age.

Use.—Coat over the iodine to a rose red and then over this mixture to a purple or slate; recoat over the first about one fourth as long as first coating.

Gurneys American Compound.—Of this compound there are two combinations, one for use, when the temperature of the atmosphere is above 65 or 70 deg., and the other at a lower temperature. The first is called No. 1, the second No. 2.

No. 1 is prepared by placing hydrate of lime in a bottle, say to three quarts of the hydrate of lime, add one ounce of pulverized burnt alum, and as much chloride of lime as can be put on a quarter of a dollar, and from 15 to 30 grains of dry pulverized iodine, or enough to change the color of the hydrate of lime, to the slightest possible tinge of yellow. There had better be less than carry the color to a deeper shade. The object of using the iodine is to form a compound with bromine that is not so volatile as the bromine itself. No matter how little iodine is combined with the bromine, the vapors possess their relative proportion; hence, only enough iodine to prevent "flaring," or as it is often termed a "scum-coating," is used. The iodine should be thoroughly combined with the lime, which will take about one or two days. Should add bromine the same as in bromide of lime, until the compound assumes a light red color.

No. 2 is prepared in the same manner as No. 1, except the addition of the iodine, which is omitted.

Use.—No. 1. Coat over the iodine to a bright yellow color, then over the compound, No. 1, to red color, recoat over iodine, about one sixth as long, as the time occupied in first coating.

No. 2. Coat over iodine same as above, except recoat over the iodine about one fourth to one half as long as first coating.

Dry Quick, No. 1.—Bromide of Lime and Starch.—The following compound forms an excellent accelerator, and is used by many. It is claimed for this preparation, that it will hold the bromine longer than others where starch is not employed. As regards this claim we do not think it can be substantiated. Our experience in practice has led us to the conclusion that there is no great difference as respects durability, but there is some little difference as regards the tone of the impressions produced by its use.

To one quart of hydrate of lime add one quart of finely pulverized starch. To this mixture add bromine, until it assumes a deep yellow or pink color.

Starch may be added to any of the dry mixtures.

Use.—Coat over the iodine to a deep yellow, then over this quick to a red color, recoat about one sixth of the time of first coating.

I will here again remark, that the exact color of the coating is not essentially provided a proper proportion is preserved.

I have never seen it stated, though it be a fact worthy of note, that a proportionate time for coating over the iodine and accelerator, will not answer. For example: if a plate exposed to the vapor of iodine be perfectly coated in sixteen seconds, and then exposed to an accelerator, (not having iodine in its combination) receives its coating in four seconds, it will be found that a proper proportionate coating cannot be preserved by adopting, a proportion of time, but on the contrary, the time will diminish; for exposure over the accelerator, as in the above example, if it be desired to coat the plate with twice as much iodine as in the above example, the time would be, over iodine thirty-two seconds, and over the accelerator (to possess a proper proportion) from six to seven seconds. Hence it is that many inexperienced operators, when wishing to vary their usual manner of coating, fail in producing a favorable result. They coat calculating a proportion of time when they should not.

Dry Quick, No. 2.—Bromide of Lime and Magnesia.—To one quart of hydrate of lime add one quart of magnesia, and mix them well together; add bromine same as in preparing bromide of lime; coat the same as over dry quick No. 1. This combination produces very uniform results, and is worked with much success by beginners.

Chloro-Bromide of Lime.—To the bromide of lime add chloride of bromine until the mixture becomes a pale yellow color, resembling sulphur. It should be shook well, and enough of the chloride of bromine added to bring the compound to a deep blood red color.

Use.—Coat over the iodine to a pink color, and then over the above to a red, or just changing the color. It should be remembered that accelerators containing chlorine do not admit of a great change of color of coating on the plate.

Iodide of Starch.—This mixture can be employed for coating over in warm weather, and prevent the flashing resulting at high temperatures. It may be used the same as the iodide alone.

To six ounces of finely pulverized starch, add one fourth ounce of dry iodine.

Use.—Same as the dry iodine alone.

The same combination may be made with lime, magnesia and other substances.

Concentrated Solution of Iodine for First Coating.—It may appear strange to some of our old operators that an aqueous solution of iodine can be used for coating the plate and forming the iodide of silver. It has long been a cry among most operators that it is impossible to succeed when the iodine box contains dampness. Now this is a great mistake, and we will here state that in all cases where dampness appears upon a properly prepared Daguerreotype plate, it is the result of a different temperature of the metal from the air which surrounds it. Mr. Senter, of Auburn, was the first of our operators who used a solution of iodine for coating the plate, and we several years since saw his results, which would rival the production of any other operator. A concentrated solution of iodine is prepared by putting into a common bottle two thimblesful of hyposulphite of soda and a rather larger quantity of iodine, so that there may be more than sufficient. Add to it about 40 ounces of common water (heated to 60 or 70 degrees), by little and little, moving, the bottle to warm it, for fear of breaking. After shaking it a short time, the water is rapidly and strongly colored. The solution should be poured into a bottle with a ground stopper, and when cool used for iodizing.

A solution of sufficient strength can be made by moistening or just covering the iodine with water.

Chloride of Iodine as an Accelerator.—This is probably one of the best accelerators that can be used for coating the plate for taking views; it works too slow, however, to meet the wants of the operating room, yet its use was formerly, for a long time, adhered to by some of our best professors. In producing views with this, we are successful in obtaining well-developed impressions, with a depth of tone and richness of appearance not to be met with in the productions of any other substances. I give its use as furnished me by an old and experienced operator, and published in Humphrey's Journal, vol. i. p. 180:

"As the process of using chloride of iodine may be of interest to some of our subscribers, I take pleasure in giving the following manipulation. To one ounce of chloride of iodine add two ounces of water; place this mixture in a coating-box, the same as quick stuff; coat the plate with dry iodine to a light yellow, or lemon color; then bring the coating to a deep pink over the chloride. The plate must be recoated over the dry iodine."

This combination has been very successfully used in one of our most extensive establishments in this city, and the superiority of the pictures produced by it was considered as an equivalent for the additional time required to bring out the impressions.

Chlorine as an Accelerator.—I shall here refer to but a single experiment in which I employed chlorine gas for coating the plate. I was provided with a retort, the neck of which was fitted to the jar of my coating-box, through a hole drilled for its reception. This was fitted perfectly tight in my coating-box. I placed some pure undiluted bromine water and the agents necessary for producing chlorine gas (in small quantity) in the retort. The result was that my first experiment produced an impression completely solarized in all its parts by an exposure of four seconds of time, which would have required an exposure of twenty seconds to produce a perfectly developed impression by the usual process.

Another trial immediately produced one of the finest toned impressions I ever saw, perfectly developed in one second of time.

My next two or three experiments proved total failures. I was unable to produce even a sign of an impression. By accident my retort was broken, and not being in a locality convenient to obtain another, my experiments were necessarily suspended.

My attention was not called to this subject again for several years, when I noticed an account of some similar experiments by F. A. P. Barnard and Dr. W. H. Harrington, the latter of whom is now of the firm of Dobyns & Harrington, of New Orleans.

From reading this article, I found my own difficulties explained. Too much of the chlorine gas was present in my coating jar. I would like to see some of our enterprising operators investigate this combination.

It is a singular fact, that the vapors of bromine and chlorine combining upon the iodide of silver, produce a more sensitive coating than when the two are combined in solution, as in chloride of bromine solution. Those having Humphrey's Journal at hand, can refer to vol. i. p. 142.

To use Bromine Water or other Accelerators in Hot Weather.—An excellent plan for using bromine water is as follows:

Fill a two-ounce bottle quarter full of it, and then fill the bottle with fine sand, which serves to preserve a low temperature; then place the bottle in a porous cup, same as used in the battery; fill this also with sand, and close the end with plaster of Paris. Place this in a coating-box, and it will be found to act with great uniformity and be quite permanent.

Bromide of Lime, another accelerator, can be used in the same manner, except it is, only necessary, when a solid sensitive is used, to mix it with the sand without placing it in a bottle. This method is employed with great success by a few, who have regarded it as a secret worth keeping.

A Combination, requiring the Use of only One Coating-box.—It is often wondered by beginners, why some solution requiring only one coating cannot be employed. This can be done, but the results are not so satisfactory as when two or more are employed. Such an accelerator may be produced by adding alcoholic solution of iodine to a solution of chlorate of potash, until the latter will take up no more of the former, and to each ounce, by measure of this solution, ten drops of a saturated solution of bromide in water are added. The solution of chlorate of potash is made by diluting, one part of a saturated solution of the salt with ten parts of water. The use of the chlorate is simply as a solvent of iodine.

Fats as Accelerators.—The use of fats, oils, or greasy substances, has been one of the most emphatic prohibitions about the Daguerreotype plate. Yet it has been proved that its presence in a small quantity upon the silver surface has the effect of reducing the time of exposure in the camera from two-thirds to three-fourths. An application may be made as follows: Pour sweet oil, or rub beef or mutton fat, on a common buff, which is free from all polishing powders. With this, buff a well-cleaned plate, and it will leave a scum, which should be mostly removed by using another buff, which should be clean. Coat the plate in the usual manner, and the result will be a great reduction in the time of exposure in the camera. The impression produced upon a plate so prepared presents, when coming from the vapor of mercury, a grey, scummy appearance, which, on the application of heat in gilding, does not improve; hence its use is not generally adopted.

We have instituted some investigations upon this subject, and in the present volume, we shall not refer to it further. Those wishing to learn more fully the effect of light upon organic substances will find Robert Hunt's "Researches on Light" an invaluable work.



LIGHT AND OPTICS.

CHAPTER IV.

Light—Optics—Solar Spectrum—Decomposition of Light—Light, Heat, and Actinism—Blue Paper and Color for the Walls of the Operating Room—Proportions of Light, Heat, and Actinism composing a Sunbeam—Refraction—Reflection—Lenses—Copying Spherical Aberration—Chromatic Aberration.

It is advisable that persons engaging in the Daguerreotype art should have at least a little knowledge of the general principles of light and optics. It is not the author's design here to give a full treatise on these subjects, but he only briefly refers to the matter, giving a few facts.

It has been well observed by an able writer, that it is impossible to trace the path of a sunbeam through our atmosphere without feeling a desire to know its nature, by what power it traverses the immensity of space, and the various modifications it undergoes at the surfaces and interior of terrestrial substances.

Light is white and colorless, as long as it does not come in contact with matter. When in apposition with any body, it suffers variable degrees of decomposition, resulting in color, as by reflection, dispersion, refraction, and unequal absorption.

To Sir I. Newton the world is indebted for proving the compound nature of a ray of white light emitted from the sun. The object of this work is not to engage in an extended theory upon the subject of light, but to recur only to some points of more particular interest to the photographic operator.

The decomposition of a beam of light can be noticed by exposing it to a prism. If, in a dark room, a beam of light be admitted through a small hole in a shutter, it will form a white round spot upon the place where it falls. If a triangular prism of glass be placed on the inside of the dark room, so that the beam of light falls upon it, it no longer has the same direction, nor does it form a round spot, but an oblong painted image of seven colors—red, orange, yellow, green, blue, indigo, and violet. This is called the solar spectrum, and will be readily understood by reference to the accompanying diagram, Fig. 1.

{133}

To those who are unacquainted with the theory of light (and for their benefit this chapter is given), it may be a matter of wonder how a beam of light can be divided.



This can be understood when I say, that white light is a bundle of colored rays united together, and when so incorporated, they are colorless; but in passing through the prism the bond of union is severed, and the colored rays come out singly and separately, because each ray has a certain amount of refracting (bending) power, peculiar to itself. These rays always hold the same relation to each other, as may be seen by comparing every spectrum or rainbow; there is never any confusion or misplacement.

There are various other means of decomposing {134} white light besides the prism, of which one of the principal and most interesting to the Daguerreotypist is by reflection from colored bodies. If a beam of white light falls upon a white surface, it is reflected without change; but if it falls upon a red surface, only the red ray is reflected: so also with yellow and other colors. The ray which is reflected corresponds with the color of the object. It is this reflected decomposed light which prevents the beautifully-colored image we see upon the ground glass in our cameras.



A sunbeam may be capable of three divisions—LIGHT, HEAT, and ACTINISM; the last causes all the chemical changes, and is the acting power upon surfaces prepared to receive the photographic image. The accompanying illustration, Fig. 2, will readily bring to the mind of the reader the relation of these one to another, and their intensities in the different parts of a decomposed sunbeam.

The various points of the solar spectrum are represented in the order in which they occur between A, and B, this exhibits the limits of the Newtonian spectrum, corresponding with Fig. 1. Sir John Herschel and Seebeck have shown that there exists, beyond the violet, a faint violet light, or rather a lavender to b, to which gradually becomes colorless; similarly, red light exists beyond the assigned limits of the red ray to a. The greatest amount of actinic power is shown at E opposite the violet; hence this color "exerts" the greatest amount of influence in the formation of the photographic image.

(Blue paper and blue color have been somewhat extensively used by our Daguerreotype operators in their operating rooms and skylights, in order to facilitate the operation in the camera. I fancy, however, that this plan cannot be productive of as much good as thought by some, from the fact, that the light falling upon the subject, and then reflected into the camera, is, coming through colorless glass, not affected by such rays as may be reflected from the walls of the operating room; and even if it were so, I conceive that it would be injurious, by destroying the harmony of shadows which might otherwise occur.) The greatest amount of white light is at C; the yellow contains less of the chemical power than any other portion of the solar spectrum. It has been found that the most intense heat is at the extreme red, b.

Artificial lights differ in their color; the white light of burning charcoal, which is the principal light from candles, oil and gas, contains three rays—red, yellow, and blue. The dazzling light emitted from lime intensely heated, known as the Drummond light, gives the colors of the prism almost as bright as the solar spectrum.

If we expose a prepared Daguerreotype plate or sensitive paper to the solar spectrum, it will be observed that the luminous power (the yellow) occupies but a small space compared with the influence of heat and chemical power. R. Hunt, in his Researches on Light, has presented the following remarks upon the accompanying illustration:



"If the linear measure, or the diameter of a circle which shall include the luminous rays, is 25, that of the calorific spectrum will be 42.10, and of the chemical spectrum 55.10. Such a series of circles may well be used to represent a beam from the sun, which may be regarded as an atom of Light, surrounded with an invisible atmosphere of Heat, and another still more extended, which possesses the remarkable property of producing chemical and molecular change.

A ray of light, in passing obliquely through any medium of uniform density, does not change its course; but if it should pass into a denser body, it would turn from a straight line, pursue a less oblique direction, and in a line nearer to a perpendicular to the surface of that body. Water exerts a stronger refracting power than air; and if a ray of light fall upon a body of this fluid its course is changed, as may be seen by reference to Fig. 4.



It is observed that it proceeds in a less oblique direction (towards the dotted line), and, on passing on through, leaves the liquid, proceeding in a line parallel to that at which it entered. It should be observed that at the surface of bodies the refractive power is exerted, and that the light proceeds in a straight line until leaving the body. The refraction is more or less, and in all cases in proportion as the rays fall more or less obliquely on the refracting surface. It is this law of optics which has given rise to the lenses in our camera tubes, by which means we are enabled to secure a well-delineated representation of any object we choose to picture.

When a ray of light passes from one medium to another, and through that into the first again, if the two refractions be equal, and in opposite directions, no sensible effect will be produced.

The reader may readily comprehend the phenomena of refraction, by means of light passing through lenses of different curves, by reference to the following diagrams:—



Fig 5 represents a double-convex lens, Fig. 6 a double-concave, and Fig. 7 a concavo-convex or meniscus. By these it is seen that a double-convex lens tends to condense the rays of light to a focus, a double-concave to scatter them, and a concavo-convex combines both powers.

If parallel rays of light fall upon a double-convex lens, D D, Fig. 8, they will be refracted (excepting such as pass directly through the centre) to a point termed the principal focus.



The lines A B C represent parallel rays which pass through the lens, D D, and meet at F; this point being the principal focus, its distance from the lens is called the focal length. Those rays of light which are traversing a parallel course, when they enter the lens are brought to a focus nearer the lens than others. Hence the difficulty the operator sometimes experiences by not being able to "obtain a focus," when he wishes to secure a picture of some very distant objects; he does not get his ground glass near enough to the lenses. Again, the rays from an object near by may be termed diverging rays. This will be better comprehended by reference to Fig. 9, where it will be seen that the dotted lines, representing parallel rays, meet nearer the lenses than those from the point A. The closer the object is to the lenses, the greater will be the divergence. This rule is applicable to copying. Did we wish to copy a 1/6 size Daguerreotype on a 1/16 size plate, we should place it in such a position to the lenses at A that the focus would be at F, where the image would be represented at about the proper size. Now, if we should wish to copy the 1/6 size picture, and produce another of exactly the same dimensions, we have only to bring it nearer to the lenses, so that the lens D E shall be equi-distant from the picture and the focus, i. e. from A to B. The reason of this is, that the distance of the picture from the lens, in the last copy, is less than the other, and the divergence has increased, throwing, the focus further from the lens."



These remarks have been introduced here as being important for those who may not understand the principles of enlarging or reducing pictures in copying.

I would remark that the points F and A, in Fig. 9, are termed "conjugate foci."

If we hold a double-convex lens opposite any object, we find that an inverted image of that object will be formed on a paper held behind it. To illustrate this more clearly, I will refer to the following woodcut:



"If A B C is an object placed before a convex lens, L L, every point of it will send forth rays in all directions; but, for the sake of simplicity, suppose only three points to give out rays, one at the top, one at the middle, and one at the bottom; the whole of the rays then that proceed from the point A, and fall on the lens L L, will be refracted and form an image somewhere on the line A G E, which is drawn direct through the centre of the lens; consequently the focus E, produced by the convergence of the rays proceding from A, must form an image of A, only in a different relative position; the middle point of C being in a direct line with the axis of the lens, will have its image formed on the axis F, and the rays proceeding from the point B will form an image at D; so that by imagining luminous objects to be made up of all infinite number of radiating points and the rays from each individual point, although falling on the whole surface of the lens, to converge again and form a focus or representation of that point from which the rays first emerged, it will be very easy to comprehend how images are formed, and the cause of those images being reversed.

"It must also be evident, that in the two triangles A G B and D G E, that E D, the length of the image, must be to A B, the length of the object, as G D, the distance of the image, is to G B, the distance of the object from the lens.

It will be observed that in the last cut the image produced by the lens is curved. Now, it would be impossible to produce a well-defined image from the centre to the edge upon a plain surface; the outer edges would be misty, indistinct, or crayon-like. The centre of the image might be represented clear and sharp on the ground glass, yet this would be far from the case in regard to the outer portions. This is called spherical aberration, and to it is due the want of distinctness which is frequently noticed around the edges of pictures taken in the camera. To secure a camera with a flat, sharp, field, should be the object of every operator; and, in a measure, this constitutes the great difference in cameras manufactured in this country.

Spherical aberration is overcome by proper care in the formation of the lens: "It can be shown upon mathematical data that a lens similar to that given in the following diagram—one surface of which is a section of an ellipse, and the other of a circle struck from the furthest of the two foci of that ellipse—produces no aberration.

"At the earliest period of the employment of the camera obscura, a double-convex lens was used to produce the image; but this form was soon abandoned, on account of the spherical aberration so caused. Lenses for the photographic camera are now always ground of a concavo-convex form, or meniscus, which corresponds more nearly to the accompanying diagram."



Chromatic Aberration is another difficulty that opticians have to contend with in the manufacturing of lenses. It will be remembered, that in a former page (133) a beam of light is decomposed by passing through a glass prism giving seven distinct colors—red, orange, yellow, green, blue, indigo and violet.

Now, as has been said before, the dissimilar rays having an unequal degree of refrangibility, it will be impossible to obtain a focus by the light passing through a double-convex lens without its being fringed with color. Its effect will be readily understood by reference to the accompanying cut.



If L L be a double convex-lens, and R R R parallel rays of white light, composed of the seven colored rays, each having a different index of refraction, they cannot be refracted to one and the same point; the red rays, being the least refrangible, will be bent to r, and the violet rays, being the most refrangible, to v: the distance v r constitutes the chromatic aberration, and the circle, of which the diameter is a l, the place or point of mean refraction, and is called the circle of least aberration. If the rays of the sun are refracted by means of a lens, and the image received on a screen placed between C and o, so as to cut the cone L a l L, a luminous circle will be formed on the paper, only surrounded by a red border, because it is produced by a section of the cone L a l L, of which the external rays L a L l, are red; if the screen be moved to the other side of o, the luminous circle will be bordered with violet, because it will be a section of the cone M a M l, of which the exterior rays are violet. To avoid the influence of spherical aberration, and to render the phenomena of coloration more evident, let an opaque disc be placed over the central portion of the lens, so as to allow the rays only to pass which are at the edge of the glass; a violet image of the sun will then be seen at v, red at r, and, finally, images of all the colors of the spectrum in the intermediate space; consequently, the general image will not only be confused, but clothed with prismatic colors."

To overcome the difficulty arising from the chromatic aberration, the optician has only to employ a combination of lenses of opposite focal length, and cut from glass possessing different refrangible powers, so that the rays of light passing through the one are strongly refracted, and in the other are bent asunder again, reproducing white light.

To the photographer one of the most important features, requiring his particular attention, is, that he be provided with a good lens. By the remarks given in the preceding pages, he will be enabled, in a measure, to judge of some of the difficulties to which he is occasionally subjected. We have in this country but two or three individuals who are giving their attention to the manufacture of lenses, and their construction is such, that they are quite free from the spherical or chromatic aberration.



CHAPTER V.

To make Plates for the Daguerreotype—Determining the Time of Exposure in the Camera—Instantaneous Process for Producing Daguerreotype—Galvanizing the Daguerreotype Plate—Silvering Solution—Daguerreotype without Mercury—Management of Chemicals—Hints and Cautions—Electrotyping—Crayon Daguerreotypes—Illuminated Daguerreotypes—Natural Colors in Heliography—Multiplying Daguerreotypes on one Plate—Deposit in Gilding—Practical Hints on the Daguerreotype.

TO MAKE PLATES FOR THE DAGUERREOTYPE.

I do not give the method employed by our regular plate manufacturers; this is not important, as the operator could not possibly profit by it from the fact of the great expense of manufacturing. The following will be found practical:

Procure a well planished copper plate of the required size, and well polish it, first with pumice stone and water, then with snake stone, jewelers' rouge. Plates can be purchased in a high state of preparation from the engravers. Having prepared the copper-plate, well rub it with salt and water, and then with the silvering powder. No kind answers better than that used by clock-makers to silver their dial-plates. It is composed of one part of well washed chloride of silver, five parts of cream of tartar, and four parts of table salt. This powder must be kept in a dark vessel, and in a dry place. For a plate six inches by five, as much of this composition as can be taken up on a shilling is sufficient. It is to be laid in the centre of the copper, and the figures being wetted, to be quickly rubbed over every part of the plate, adding occasionally a little damp salt. The copper being covered with the silvering is to be speedily well washed in water, in which a little soda is dissolved, and as soon as the surface is of a fine silvery whiteness, it is to be dried with a very clean warm cloth. In this state the plates may be kept for use. The first process is to expose the plate to the heat of a spirit flame, until the silvered surface becomes of a well-defined golden-yellow color; then, when the plate is cold, take a piece of cotton, dipped in very dilute nitric acid, and rub lightly over it until the white hue is restored, and dry it with very soft clean cloths. A weak solution of the hydriodate of potash, in which a small portion of iodine is dissolved, is now passed over the plate with a wide camel's hair brush. The silver is thus converted, over its surface, into an ioduret of silver; and in this state it is exposed to light, which blackens it. When dry, it is to be again polished, either with dilute acid or a solution of carbonate of soda, and afterwards with dry cotton, and the smallest possible portion of prepared chalk: by this means a surface of the highest polish is produced. The rationale of this process is, in the first place, the heat applied dries off any adhering acid, and effects more perfect union between the copper and silver, so as to enable it to bear the subsequent processes. The first yellow surface appears to be an oxide of silver with, possibly, a minute quantity of copper in combination, which being removed leaves a surface chemically pure.

Another Method.—The best and simplest mode with which we are acquainted is to divide an earthenware vessel with a diaphragm: one side should be filled with a very dilute solution of sulphuric acid, and the other with either a solution of ferroprussiate of potash, or muriate of soda, saturated with chloride of silver. The copper plate, varnished on one side, is united, by means of a copper wire, with a plate of zinc. The zinc plate being immersed in the acid, and the copper in the salt, a weak electric current is generated, which precipitates the silver in a very uniform manner over the entire surface.

Another Method.—A piece of brass or of polished copper, brass is preferred, is perfectly planished and its surface made perfectly clean. A solution of nitrate of silver, so weak that the silver is precipitated slowly, and a brownish color, on the brass, is laid uniformly over it, "at least three times," with a camel's hair pencil. After each application of the nitrate, the plate should be rubbed gently in one direction, with moistened bitartrate of potassa, applied with buff. This coat of silver receives a fine polish from peroxide of iron and buff. Proofs are said to have been taken on it, comparable with those obtained on French plates.

M. SOLIEL'S PROCESS FOR DETERMINING THE TIME OF EXPOSURE IN THE CAMERA.

M. Soliel has proposed the use of the chloride of silver to determine the time required to produce a good impression on the iodated plate in the camera. His method is to fix at the bottom of a tube, blackened within, a piece of card, on which chloride of silver, mixed with gum or dextrine, is spread. The tube thus disposed is turned from the side of the object of which we wish to take the image, and the time that the chloride of silver takes to become of a greyish slate color will be the time required for the light of the camera to produce a good effect on the iodated silver.

INSTANTANEOUS PROCESS FOR PROCURING DAGUERREOTYPES.

The following method of producing Daguerreotypes has by some been named as above. Most experienced operators have been long acquainted with the effect of the vapor of ammonia upon the chemically coated plate. I will here insert Mr. W. H. Hewett's plan of proceeding. This gentleman, in referring to it (published in 1845), says:

"This improvement consists in using the vapor of ammonia, as an object to accelerate the action of light upon the plate. The effect is produced upon a simple iodized plate, but still more upon a plate prepared in the ordinary way, with both iodine and bromine. By this means, the author obtained impressions instantaneously in the sunshine, and in five to ten seconds in a moderate light; and he hopes to be able to take moving objects. It can be applied by exposing the prepared plate over a surface of water, to which a few drops of ammonia have been added (sufficient to make it smell of ammonia); or the vapor can be introduced into the camera during the action. In fact, the presence of ammonia, in the operating-room, appears to have a good effect, as it also neutralizes the vapors of iodine and bromine that may be floating about, and which are so detrimental to the influences of light upon the plate."

GALVANIZING THE DAGUERREOTYPE PLATE.

In consideration of the importance of galvanized plates, I shall endeavor to give as plain and concise a manner of manipulation as possible. For some time it was a question among the operators generally, as to the beneficial result of electrotyping, the Daguerreotype plate, but for a few years past our first operators have found it a fact, that a well electro-silvered surface is the best for producing a portrait by the Daguerreotype.

From my own experiments, I have found that a plate, by being galvanized, can be rendered more sensitive to the operation of the light in proportion of one to five, viz.: if a plate as furnished by the market, be cleaned, polished, coated and exposed in the camera, if the required time to freely develop an impression be ten seconds, a similar plate prepared in like manner and galvanized, will produce an equally well-defined image in eight seconds. In connection with this subject, there is one fact worthy of notice; a plate with a very heavy coating of pure silver, will not produce an equally developed image, as a plate with a thinner coating, hence the thin coating, providing it entirely covers the surface, is the best, and is the one most to be desired. The experiment is plain and simple. Let the slate receive a heavy or thick coating by the electrotype, then polish, coat, expose in the usual manner, and the result will be a flat, ashy, indistinct impression; when, on the other hand, the thin coating will produce a bright, clear and distinct image, with all the details delineated.

The style of battery best for the purpose has been, and now is, a question of dispute among operators; some preferring the Daniell battery to Smee's. Some claim the superiority of the first from its uniformity of action; others, of the latter, for its strength. I consider either good, and for the inexperienced would prefer the Daniell. This is more simple in its construction, while it has certainty in action. The more skillful electrotyper would prefer Smee's, and this is the one most generally in use. I would remark that the plan of galvanizing plates should be followed by every operator, and when once thoroughly tested, no one will abandon it.

SILVERING SOLUTION.

To any desired quantity of chloride of silver in water add, little by little, cyanide of potassium, shaking well at each addition, until all the cyanide is dissolved. Continue this operation, and add the cyanide, until all the precipitate is taken up and held in solution.

This solution is now ready for the plate-cup. Enough water may be added to cover any sized plate when held perpendicular in the cup. The strength of the solution may be kept up by occasionally adding the chloride of silver and cyanide of potassium. There should alway be a very little excess of the cyanide.

The plate should be well cleaned and buffed, and the solution well stirred before it is immersed. Care should be observed to keep the solution clean, and allow no particle of dust to come in contact with the surface of the plate. The plate is now to be attached to the pole of the battery.

After remaining a short time, it assumes a blue color; take it out, rinse freely with pure water, then dry with a spirit lamp, and it is ready for buffing. Buff and coat in the usual manner. Some operators are in the practice of immersing the plate in the solution and buffing twice. This additional silvering is no improvement wherever there has been a proper first coating.

Sometimes the operator is troubled with streaks or scum on the plate. This may arise from three causes, all of which experience must teach the experimenter to avoid; first, too great an excess of cyanide in the solution; second, a lack of silver; third, the current too strong. Another annoyance arises from the solution being dirty and the dirt collecting on the surface. When this is the case, the dirt is sure to come in contact with the surface of the plate as it is plunged into the solution, and the result is a scum that it is difficult to dispose of. This can be prevented only by frequent filtering. One thing should always be borne in mind in electrotyping Daguerreotype plates—that in order to secure a perfectly coated surface, the plate should be perfectly cleaned. In this point, many who have tried the electrotype process have failed, attributing their ill success to other than the proper cause.



DAGUERREOTYPES WITHOUT MERCURY.

The following process possesses some interest, and is worthy a trial from operators. M. Natterer, of Vienna, discovered a process for obtaining proofs on iodized plates with the chloride of sulphur, without the use of mercury. A plate of silver is iodized in the usual manner, and then placed on the top of a vessel six or eight inches high, having at the bottom, in a small cup, a few drops of chloride of sulphur; it should remain exposed to the action of the vapor until the sombre yellow color is changed to a red, after which it is brought to a focus in the camera, where it is exposed to the light in the camera, for about the time necessary to produce an ordinary daguerreotype. The plate is then taken out and examined in the camera by the light of a candle. It often occurs that no trace of the image is as yet perceptible, but if the plate is heated by placing over a spirit lamp the unprepared side, or if left for some time in the dark, or, lastly, if exposed only a few seconds to a weak, dimmed light, the positive picture then appears with all its shades. Of these three modes of bringing out the image, the second is superior to the others.

MANAGEMENT OF CHEMICALS.

It is necessary, first of all, to know that you have a chemical which is capable of producing good results when in skillful hands. For this reason it is best to prepare your own quick, after some formula which is known to be good. Those quick-stuffs which contain chloride of iodine are noted for their depth of tone while they probably operate with less uniformity than those which are destitute of it. For operating under ordinary circumstances, especially with an inferior light, probably no accelerator is more quick and sure than Wolcott's. It also produces a very fine, white pleasing picture, though lacking that depth of impression so much to be desired. The dry quick operates with surety, and its use is simple and easy, producing an impression much like Wolcott's. For those having a good and permanent light, however, we would recommend a chemical giving more body to the impression.

There is a class of accelerators called sensitives, claiming to work in from three to ten seconds, which, however, will be found very little, if any, more sensitive than this. We frequently work it with the ordinary coating in twelve and fifteen seconds. The manner in which the sensitives are worked is by coating very light. In this way, a flat, shallow picture is obtained in a few seconds; and the same can be done with any of the more volatile quicks.

It is a fact not generally known, that a plate coated in a light chemical room is more sensitive than when coated in darkness. By admitting a free, uniform light, and exposing the plate to it a few seconds after coating, then timing short in the camera, a very light, clear impression is obtained. The time in the camera is reduced in proportion to the previous action of light. The shades, of course, are destroyed, and the tone injured; still, for taking children, we have succeeded better by this method than by the use of "sensitives." The discovery of this principle was accidental, while operating where the direct ray s of the sun, entering the window just before sunset, fell on the curtain of our dark room, rendering it very light within.

The selection of iodine is not unimportant. Reject, at once, that which has anything like a dull, black, greasy appearance; and select that which is in beautiful large crystalline scales, of a purple color, and brilliant steel lustre.

Solarization, and general blueness of all the light parts of the picture, were formerly great obstacles to success, though now scarcely thought of by first-class artists. Beginners in the art, however, are still apt to meet with this difficulty. It is occasioned by dampness in the iodine box, which causes the plate to become coated with a hydro-iodide of silver, instead of the iodide. The remedy is in drying your iodine. If in summer, you can open your box and set it in sunshine a few minutes; or if in winter, set it under a stove a short time. The true method, however, is to dry it by means of the chloride of calcium. It has such a remarkable affinity for water, that a small fragment placed in the open air, even in the dryest weather, soon becomes dissolved.

Take one or two ounces of this chemical, heat it in the drying bath, or in a hot stove, to perfect dryness; place it in a small glass toy dish, or large watch crystal, and set it in the centre of your iodine box. Take this out and heat to dryness every morning. Adopt this process, and with your mercury at a high temperature, you will never be troubled with blue pictures.

Young operators are apt to impute all want of success in operating to their chemicals, even though the cause is quite as likely to be elsewhere. Failure is quite likely to occur from dampness in the buffs, or in the polish; it is therefore necessary to be constantly on the guard in this quarter. With a view to this, always scrape your buffs with a dull knife, or with one blade of your shears, the first thing in the morning, and after brushing them thoroughly, dry them, either in the sun, by a stove, or in the buff-dryer. It is equally important that the polish and the brush should be kept dry.

Want of success may arise from vapors of iodine or bromine in the camera box, mercury bath, or even in the buffs. It is incredible how small a quantity of these vapors will affect the effect of light when coming in contact with the plate, after or during the exposure in the camera. It is therefore necessary to be cautious not to mix chemicals, nor open your boxes or bottles in your room, but take them out to do it. Never hurry the operation through from lack of confidence in the result. The fact of anything being out of order, forms no excuse for slighting the process. If unsuccessful, do not pursue the same course every trial, but vary with a view to detect the cause of the difficulty.

In case of a long series of failures, institute a regular course of investigation, after this manner, commencing where the trouble is most likely to occur:

1. Are the plates well cleaned?

2. Is the iodine dry? If the impressions come out blue, you may rest assured it is not. Take out the iodine, wipe and dry the box, and dry the calcium.

3. Is the quick battery of the right strength? If dry, it must change the plate in from six to fifteen seconds. If any of the chloride of iodine class, it may vary from five seconds to a minute. Begin by coating light, and increase on each trial, observing the effect. If the light side of the picture seems loth to come out, and shows no contrast with the dark side, it is to be inferred that your battery is too strong, and must be reduced with water or set out in the open air for a few minutes, with the lid off. If working an old battery, never renew very strong, or it will work dark and heavy. A battery, to work well, should be gradually losing strength, but never gaining. An old battery, however, may be quickened up and made to work well for some time, by adding five of six drops of sulphuric acid, repeating the quantity as often as necessary, providing always that acid be not used in manufacturing the quick.

4. Have the plates lost their sensitiveness by being many times exposed to mercury? Clean and burn them; but if French plates, burn light, or you spoil them.

5. Are the buff s dry and clean? Examine the plate critically after buffing to detect any appearance of scum or film on the surface. If so, the longer you buff the more it shows. Scrape and dry the buffs thoroughly.