(2) Electrical theory.—The second, or electrical, theory supposes that the visual impulse is the concomitant of an electrical impulse; that an electrical current is generated in the retina under the incidence of light, and that this is transmitted to the brain by the optic nerve. There is much to be said in favour of this view, for it is an undoubted fact, that light gives rise to retinal currents, and that, conversely, an electrical current suitably applied causes the sensation of light.

Retinal currents.—Holmgren, Dewar, McKendrick, Kuhne, Steiner, and others have shown that illumination produces electric variation in a freshly excised eye. About this general fact of the electrical response there is a widespread agreement, but there is some difference of opinion as regards the sign of this response immediately on the application, cessation, and during the continuance of light. These slight discrepancies may be partly due to the unsatisfactory nomenclature—as regards use of terms positive and negative—hitherto in vogue and partly also to the differing states of the excised eyes observed.

Waller, in his excellent and detailed work on the retinal currents of the frog, has shown how the sign of response is reversed in the moribund condition of the eye.

As to the confusion arising from our present terminology, we must remember that the term positive or negative is used with regard to a current of reference—the so-called current of injury.



When the two galvanometric contacts are made, one with the cut end of the nerve, and the other on the uninjured cornea, a current of injury is found which in the eye is from the nerve to the retina. In the normal freshly excised eye, the current of response due to the action of light on the retina is always from the nerve, which is not directly stimulated by light, to the retina, that is, from the less excited to the more excited (fig. 95). This current of response flows, then, in the same direction as the existing current of reference—the current of injury—and may therefore be called positive. Unfortunately the current of injury is very often apt to change its sign; it then flows through the eye from the cornea to the nerve. And now, though the current of response due to light may remain unchanged in direction, still, owing to the reversal of the current of reference, it will appear as negative. That is to say, though its absolute direction is the same as before, its relative direction is altered.

I have already advocated the use of the term positive for currents which flow towards the stimulated, and negative for those whose flow is away from the stimulated. If such a convention be adopted, no confusion can arise, even when, as in the given cases, the currents of injury undergo a change of direction.

Normal response positive.—The normal effect of light on the retina, as noticed by all the observers already mentioned, is a positive variation, during exposure to light of not too long duration. Cessation of light is followed by recovery. On these points there is general agreement amongst investigators. Deviations are regarded as due to abnormal conditions of the eye, owing to rough usage, or to the rapid approach of death. For just as in the dying plant we found occasional reversals from negative to positive response, so in the dying retina the response may undergo changes from the normal positive to negative.

The sign of response, as we have already seen in numerous cases, depends very much on the molecular condition of the sensitive substance, and if this condition be in any way changed, it is not surprising that the character of the response should also undergo alteration.

Unlike muscle in this, successive retinal responses exhibit little change, for, generally speaking, fatigue is very slight, the retina recovering quickly even under strong light if the exposure be not too long. In exceptional cases, however, fatigue, or its converse, the staircase effect, may be observed.

Inorganic response under the stimulus of light.—It may now be asked whether such a complex vital phenomenon as retinal response could have its counterpart in non-living response. Taking a rod of silver, we may beat out one end into the form of a hollow cup, sensitising the inside by exposing it for a short time to vapour of bromine. The cup may now be filled with water, and connection made with a galvanometer by non-polarisable electrodes. There will now be a current due to difference between the inner surface and the rod. This may be balanced, however, by a compensating E.M.F.



We have thus an arrangement somewhat resembling the eye, with a sensitive layer corresponding to the retina, and the less sensitive rod corresponding to the conducting nerve-stump (fig. 96, a).

The apparatus is next placed inside a black box, with an aperture at the top. By means of an inclined mirror, light may be thrown down upon the sensitive surface through the opening.

On exposing the sensitive surface to light, the balance is at once disturbed, and a responsive current of positive character produced. The current, that is to say, is from the less to the more stimulated sensitive layer. On the cessation of light, there is fairly quick recovery (fig. 96, b).

The character and the intensity of E.M. variation of the sensitive cell depend to some extent on the process of preparation. The particular cell with which most of the following experiments were carried out usually gave rise to a positive variation of about .008 volt when acted on for one minute by the light of an incandescent gas-burner which was placed at a distance of 50 cm.

Typical experiment on the electrical effect induced by light.—This subject of the production of an electrical current by the stimulus of light would appear at first sight very complex. But we shall be able to advance naturally to a clear understanding of its most complicated phenomena if we go through a preliminary consideration of an ideally simple case. We have seen, in our experiments on the mechanical stimulation of, for example, tin, that a difference of electric potential was induced between the more stimulated and less stimulated parts of the same rod, and that an action current could thus be obtained, on making suitable electrolytic connections. Whether the more excited was zincoid or cuproid depended on the substance and its molecular condition.

Let us now imagine the metal rod flattened into a plate, and one face stimulated by light, while the other is protected. Would there be a difference of potential induced between the two faces of this same sheet of metal?

Let two blocks of paraffin be taken and a large hole drilled through both. Next, place a sheet of metal between the blocks, and pour melted paraffin round the edge to seal up the junction, the two open ends being also closed by panes of glass. We shall have then two compartments separated by the sheet of metal, and these compartments may be filled with water through the small apertures at the top (fig. 97, a).



The two liquid masses in the separated chambers thus make perfect electrolytic contacts with the two faces A and B of the sheet of metal. These two faces may be put in connection with a galvanometer by means of two non-polarisable electrodes, whose ends dip into the two chambers. If the sheet of metal have been properly annealed, there will now be no difference of potential between the two faces, and no current in the galvanometer. If the two faces are not molecularly similar, however, there will be a current, and the electrical effects to be subsequently described will act additively, in an algebraical sense. Let one face now be exposed to the stimulus of light. A responsive current will be found to flow, from the less to the more stimulated face, in some cases, and in others in an opposite direction.

It appears at first very curious that this difference of electric potential should be maintained between opposite faces of a very thin and highly conducting sheet of metal, the intervening distance between the opposed surfaces being so extremely small, and the electrical resistance quite infinitesimal. A homogeneous sheet of metal has become by the unequal action of light, molecularly speaking, heterogeneous. The two opposed surfaces are thrown into opposite kinds of electric condition, the result of which is as if a certain thickness of the sheet, electrically speaking, were made zinc-like, and the rest copper-like. From such unfamiliar conceptions, we shall now pass easily to others to which we are more accustomed. Instead of two opposed surfaces, we may obtain a similar response by unequally lighting different portions of the same surface. Taking a sheet of metal, we may expose one half, say A, to light, the other half, B, being screened. Electrolytic contacts are made by plunging the two limbs in two vessels which are in connection with the two non-polarisable electrodes E and E' (fig. 98, a). On illumination of A and B alternately, we shall now obtain currents flowing alternately in opposite directions.



Just as in the strain cells the galvanometer contact was transferred from the electrolytic part to the metallic part of the circuit, so we may next, in an exactly similar manner, cut this plate into two, and connect these directly to the galvanometer, electrolytic connection being made by partially plunging them into a cell containing water. The posterior surfaces of the two half-plates may be covered with a non-conducting coating. And we arrive at a typical photo-electric cell (fig. 98, b). These considerations will show that the eye is practically a photo-electric cell.



We shall now give detailed experimental results obtained with the sensitive silver-bromide cell, and compare its response-curve with those of the retina. A series of uniform light stimuli gives rise to uniform responses, which show very little sign of fatigue. How similar these response-curves are to those of the retina will be seen from a pair of records given below, where fig. 99 shows responses of frog's retina, and fig. 100 gives the responses obtained with the sensitive silver cell (fig. 100).

It was said that the responses of the retina are uniform. This is only approximately true. In addition to numerous cases of uniform responses, Waller finds instances of 'staircase' increase, and its opposite, slight fatigue. In the record here given of the silver cell, the staircase effect is seen at the beginning, and followed by slight fatigue. I have other records where for a very long time the responses are perfectly uniform, there being no sign of fatigue.



Another curious phenomenon sometimes observed in the response of retina is an occasional slight increase of response immediately on the cessation of light, after which there is the final recovery. An indication of this is seen in the second and fourth curves in fig. 99. Curiously enough, this abnormality is also occasionally met with in the responses of the silver cell, as seen in the first two curves of fig. 100. Other instances will be given later.



CHAPTER XVIII

INORGANIC RESPONSE—INFLUENCE OF VARIOUS CONDITIONS ON THE RESPONSE TO STIMULUS OF LIGHT

Effect of temperature—Effect of increasing length of exposure—Relation between intensity of light and magnitude of response—After-oscillation—Abnormal effects: (1) preliminary negative twitch; (2) reversal of response; (3) transient positive twitch on cessation of light; (4) decline and reversal—Resume.

We shall next proceed to study the effect, on the response of the sensitive cell, of all those conditions which influence the normal response of the retina. We shall then briefly inquire whether even the abnormalities sometimes met with in retinal responses have not their parallel in the responses given by the inorganic.



Effect of temperature.—It has been found that when the temperature is raised above a certain point, retinal response shows rapid diminution. On cooling, however, response reappears, with its original intensity. In the response given by the sensitive cell, the same peculiarity is noticed. I give below (fig. 101, a) a set of response-curves for 20 deg. C. These responses, after showing slight fatigue, became fairly constant. On raising the temperature to 50 deg. C. response practically disappeared (101, b). But on cooling to the first temperature again, it reappeared, with its original if not slightly greater intensity (fig. 101, c). A curious point is that while in record (a), before warming, slight fatigue is observed, in (c), after cooling, the reverse, or staircase effect, appears.



Effect of increasing length of exposure.—If the intensity of light be kept constant, the magnitude of response of the sensitive cell increases with length of exposure. But this soon reaches a limit, after which increase of duration does not increase magnitude of effect. Too long an exposure may however, owing to fatigue, produce an actual decline.

I give here two sets of curves (fig. 102) illustrating the effect of lengthening exposure. The intensities of light in the two cases are as 1 to 4. The incandescent burner was in the two cases at distances 50 and 25 cm. respectively. It will be observed that beyond eight seconds' exposure the responses are approximately uniform. Another noticeable fact is that with long exposure there is an after-oscillation. This growing effect with lengthening exposure and attainment of limit is exactly paralleled by responses of retina under similar conditions.

Relation between intensity of light and magnitude of response.—In the responses of retina, it is found that increasing intensity of light produces an increasing effect. But the rate of increase is not uniform: increase of effect does not keep pace with increase of stimulus. Thus a curve giving the relation between stimulus and response is concave to the axis which represents the stimulus.

The same is true of the sensation of light. That is to say, within wide limits, intensity of sensation does not increase so rapidly as stimulus.

This particular relation between stimulus and effect is also exhibited in a remarkable manner by the sensitive cell. For a constant source of light I used an incandescent burner, and graduated the intensity of the incident light by varying its distance from the sensitive cell. The intensity of light incident on the cell, when the incandescent burner is at a distance of 150 cm., has been taken as the arbitrary unit. In order to make allowance for the possible effects of fatigue I took two successive series of responses (fig. 103). In the first, records were taken with intensities diminishing from 7 to 1, and immediately afterwards increasing from 1 to 7, in the second.



TABLE GIVING RESPONSE TO VARYING INTENSITIES OF LIGHT

(The intensity of an incandescent gas-burner at a distance of 150 cm. is taken as unit.)

- - Response Response Intensity (Light (Light Mean Value in volt of Light diminishing) increasing) - - 7 43 39 41 63.0 x 10^{-} volt 5 31 29 30 46.1 x " 3 18.5 17.5 18 27.7 x " 1 10 9 9.5 14.6 x " - -

As the zero point was slightly shifted during the course of the experiment, the deflection in each curve was measured from a line joining the beginning of the response to the end of its recovery. A mean deflection, corresponding to each intensity, was obtained by taking the average of the descending and ascending readings. The two sets of readings did not, however, vary to any marked extent.

The deflections corresponding to the intensities 1, 3, 5, 7, are, then, as 9.5 to 18, to 30, to 41. If the deflections had been strictly proportionate to the intensities of light stimulus they would have been as 9.5 to 28.5, to 47.5, to 66.5.



In another set of records, with a different cell, I obtained the deflections of 6, 10, 13, 15, corresponding to light intensities of 3, 5, 7, and 9.

The two curves in fig. 104, giving the relation between response and stimulus, show that in the case of inorganic substances, as in the retina (Waller), magnitude of response does not increase so rapidly as stimulus.

After-oscillation.—When the sensitive surface is subjected to the continued action of light, the E.M. effect attains a maximum at which it remains constant for some time. If the exposure be maintained after this for a longer period, there will be a decline, as we found to be the case in other instances of continued stimulation. The appearance of this decline, and its rapidity, depends on the particular condition of the substance.

When the sensitive element is considerably strained by the action of light, and if that light be now cut off, there is a rebound towards recovery and a subsequent after-oscillation. That is to say, the curve of recovery falls below the zero point, and then slowly oscillates back to the position of equilibrium. We have already seen an instance of this in fig. 102. Above is given a series of records showing the appearance of decline, from too long-continued exposure and recovery, followed by after-oscillation on the cessation of light (fig. 105). Certain visual analogues to this phenomenon will be noticed later.



Abnormal effects.—We have already treated of all the normal effects of the stimulus of light on the retina, and their counterparts in the sensitive cell. But the retina undergoes molecular changes when injured, stale, or in a dying condition, and under these circumstances various complicated modifications are observed in the response.



#1. Preliminary negative twitch.#—When the light is incident on the frog's retina, there is sometimes a transitory negative variation, followed by the normal positive response. This is frequently observed in the sensitive cell (see fig. 96, b).

#2. Reversal of response.#—Again, in a stale retina, owing to molecular modification the response is apt to undergo reversal (Waller). That is to say, it now becomes negative. In working with the same sensitive cell on different days I have found it occasionally exhibiting this reversed response.

#3. Transient rise of current on cessation of light.#—Another very curious fact observed in the retina by Kuhne and Steiner is that immediately on the stoppage of light there is sometimes a sudden increase in the retinal current, before the usual recovery takes place. This is very well shown in the series of records taken by Waller (fig. 106). It will be noticed that on illumination the response-curve rises, that continued illumination produces a decline, and that on the cessation of light there is a transient rise of current. I give here a series of records which will show the remarkable similarity between the responses of the cell and retina, in respect even of abnormalities so marked as those described (fig. 107). I may mention here that some of these curious effects, that is to say, the preliminary negative twitch and sudden augmentation of the current on the cessation of light, have also been noticed by Minchin in photo-electric cells.



#4. Decline and reversal.#—We have seen that under the continuous action of light, response begins to decline. Sometimes this process is very rapid, and in any case, under continued light, the deflection falls.

(1) The decline may nearly reach zero. If now the light be cut off there is a rebound towards recovery downwards, which carries it below zero, followed by an after-oscillation (fig. 108, a).



(2) If the light be continued for a longer time, the decline goes on even below zero; that is to say, the response now becomes apparently negative. If, now, the light be stopped, there is a rebound upwards to recovery, with, generally speaking, a slight preliminary twitch downwards (fig. 108, b, c). This rebound carries it back, not only to the zero position, but sometimes beyond that position. We have here a parallel to the following observation of Dewar and McKendrick: 'When diffuse light is allowed to impinge on the eye of the frog, after it has arrived at a tolerably stable condition, the natural E.M.F. is in the first place increased, then diminished; during the continuance of light it is still slowly diminished to a point where it remains tolerably constant, and on the removal of light there is a sudden increase of the E.M. power nearly up to its original position.'[18]

(3) I have sometimes obtained the following curious result. On the incidence of light there is a response, say, upward. On the continuation of light the response declines to zero and remains at the zero position, there being no further action during the continuation of stimulus. But on the cessation or 'break' of light stimulus, there is a response downwards, followed by the usual recovery. This reminds us of a somewhat similar responsive action produced by constant electric current on the muscle. At the moment of 'make' there is a responsive twitch, but afterwards the muscle remains quiescent during the passage of the current, but on breaking the current there is seen a second responsive twitch.

Resume.—So we see that the response of the sensitive inorganic cell, to the stimulus of light, is in every way similar to that of the retina. In both we have, under normal conditions, a positive variation; in both the intensity of response up to a certain limit increases with the duration of illumination; it is affected, in both alike, by temperature; in both there is comparatively little fatigue; the increase of response with intensity of stimulus is similar in both; and finally, even in abnormalities—such as reversal of response, preliminary negative twitch on commencement, and terminal positive twitch on cessation of illumination, and decline and reversal under continued action of light—parallel effects are noticed.



We may notice here certain curious relations even in these abnormal responses (fig. 109). If the equilibrium position remain always constant, then it is easy to understand how, when the rising curve has attained its maximum, on the cessation of light, recovery should proceed downwards, towards the equilibrium position (fig. 109, a). One can also understand how, after reversal by the continued action of light, there should be a recovery upwards towards the old equilibrium position (fig. 109, b). What is curious is that in certain cases we get, on the stoppage of light, a preliminary twitch away from the zero or equilibrium position, upwards as in (c) (compare also fig. 107) and downwards as in (d) (compare also fig. 108 b).

In making a general retrospect, finally, of the effects produced by stimulus of light, we find that there is not a single phenomenon in the responses, normal or abnormal, exhibited by the retina which has not its counterpart in the sensitive cell constructed of inorganic material.

FOOTNOTES:

[18] Proc. Roy. Soc. Edin., 1873 p. 153.



CHAPTER XIX

VISUAL ANALOGUES

Effect of light of short duration—After-oscillation—Positive and negative after-images—Binocular alternation of vision—Period of alternation modified by physical condition—After-images and their revival—Unconscious visual impression.

We have already referred to the electrical theory of the visual impulse. We have seen how a flash of light causes a transitory electric impulse not only in the retina, but also in its inorganic substitute. Light thus produces not only a visual but also an electrical impulse, and it is not improbable that the two may be identical. Again, varying intensities of light give rise to corresponding intensities of current, and the curves which represent the relation between the increasing stimulus and the increasing response have a general agreement with the corresponding curve of visual sensation. In the present chapter we shall see how this electrical theory not only explains in a simple manner ordinary visual phenomena, but is also deeply suggestive with regard to others which are very obscure.

We have seen in our silver cell that if the molecular conditions of the anterior and posterior surfaces were exactly similar, there would be no current. In practice, however, this is seldom the case. There is, generally speaking, a slight difference, and a feeble current in the circuit. It is thus seen that there may be an existing feeble current, to which the effect of light is added algebraically. The stimulus of light may thus increase the existing current of darkness (positive variation). On the cessation of light again, the current of response disappears and there remains only the feeble original current.

In the case of the retina, also, it is curious to note that on closing the eye the sensation is not one of absolute darkness, but there is a general feeble sensation of light, known as 'the intrinsic light of the retina.' The effect produced by external light is superposed on this intrinsic light, and certain curious results of this algebraical summation will be noticed later.



Effect of light of short duration.—If we subject the sensitive cell to a flash of radiation, the effect is not instantaneous but grows with time. It attains a maximum some little time after the incidence of light, and the effect then gradually passes away. Again, as we have seen previously with regard to mechanical strain, the after-effect persists for a slightly longer time when the stimulus is stronger. The same is true of the after-effect of the stimulus of light. Two curves which exhibit this are given below (fig. 110). With regard to the first point—that the maximum effect is attained some time after the cessation of a short exposure—the corresponding experiment on the eye may be made as follows: at the end of a tube is fixed a glass disc coated with lampblack, on which, by scratching with a pin, some words are written in transparent characters. The length of the tube is so adjusted that the disc is at the distance of most distinct vision from the end of the tube applied to the eye. The blackened disc is turned towards a source of strong light, and a short exposure is given by the release of a photographic shutter interposed between the disc and the eye. On closing the eye, immediately after a short exposure, it will at first be found that there is hardly any well-defined visual sensation; after a short time, however, the writing on the blackened disc begins to appear in luminous characters, attains a maximum intensity, and then fades away. In this case the stimulus is of short duration, the light being cut off before the maximum effect is attained. The after-effect here is positive, there being no reversal or interval of darkness between the direct image and the after-image, the one being merely the continuation of the other. But we shall see, if light is cut off after a maximum effect is attained by long exposure, that the immediate after-image would be negative (see below). The relative persistence of after-effect of lights of different intensities may be shown in the following manner:

If a bold design be traced with magnesium powder on a blackened board and fired in a dark room, the observer not being acquainted with the design, the instantaneous flash of light, besides being too quick for detailed observation, is obscured by the accompanying smoke. But if the eyes be closed immediately after the flash, the feebler obscuring sensation of smoke will first disappear, and will leave clear the more persistent after-sensation of the design, which can then be read distinctly. In this manner I have often been able to see distinctly, on closing the eyes, extremely brief phenomena of light which could not otherwise have been observed, owing either to their excessive rapidity or to their dazzling character.[19]

After-oscillation.—In the case of the sensitive silver cell, we have seen (fig. 105), when it has been subjected for some time to strong light, that the current of response attains a maximum, and that on the stoppage of the stimulus there is an immediate rebound towards recovery. In this rebound there may be an over-shooting of the equilibrium position, and an after-oscillation is thus produced.

If there has been a feeble initial current, this oscillatory after-current, by algebraical summation, will cause the current in the circuit to be alternately weaker and stronger than the initial current.

Visual recurrence.—Translated into the visual circuit, this would mean an alternating series of after-images. On the cessation of light of strong intensity and long duration, the immediate effect would be a negative rebound, unlike the positive after-effect which followed on a short exposure.

The next rebound is positive, giving rise to a sensation of brightness. This will go on in a recurrent series.

If we look for some time at a very bright object, preferably with one eye, on closing the eye there is an immediate dark sensation followed by a sensation of light. These go on alternating and give rise to the phenomena of recurrent vision. With the eyes closed, the positive or luminous phases are the more prominent.

This phenomenon may be observed in a somewhat different manner. After staring at a bright light we may look towards a well-lighted wall. The dark phases will now become the more noticeable.

If, however, we look towards a dimly lighted wall, both the dark and bright phases will be noticed alternately.

The negative effect is usually explained as due to fatigue. That position of the retina affected by light is supposed to be 'tired,' and a negative image to be formed in consequence of exhaustion. By this exhaustion is meant either the presence of fatigue-stuffs, or the breaking-down of the sensitive element of the tissue, or both of these. In such a case we should expect that this fatigue, with its consequent negative image, would gradually and finally disappear on the restoration of the retina to its normal condition.

We find, however, that this is not the case, for the negative image recurs with alternate positive. The accepted theory of fatigue is incapable of explaining this phenomenon.

In the sensitive silver cell, we found that the molecular strain produced by light gave rise to a current of response, and that on the cessation of light an oscillatory after-effect was produced. The alternating after-effect in the retina points to an exactly similar process.

Binocular alternation of vision.—It was while experimenting on the phenomena of recurrent vision that I discovered the curious fact that in normal eyes the two do not see equally well at a given instant, but that the visual effect in each eye undergoes fluctuation from moment to moment, in such a way that the sensation in the one is complementary to that in the other, the sum of the two sensations remaining approximately constant. Thus they take up the work of seeing, and then, relatively speaking, resting, alternately. This division of labour, in binocular vision, is of obvious advantage.

As regards maximum sensation in the two retinae there is then a relative retardation of half a period. This may be seen by means of a stereoscope, carrying, instead of stereo-photographs, incised plates through which we look at light. The design consists of two slanting cuts at a suitable distance from each other. One cut, R, slants to the right, and the other, L, to the left (see fig. 111). When the design is looked at through the stereoscope, the right eye will see, say R, and the left L, the two images will appear superimposed, and we see an inclined cross. When the stereoscope is turned towards the sky, and the cross looked at steadily for some time, it will be found, owing to the alternation already referred to, that while one arm of the cross begins to be dim, the other becomes bright, and vice versa. The alternate fluctuations become far more conspicuous when the eyes are closed; the pure oscillatory after-effects are then obtained in a most vivid manner. After looking through the stereoscope for ten seconds or more, the eyes are closed. The first effect observed is one of darkness, due to the rebound. Then one luminous arm of the cross first projects aslant the dark field, and then slowly disappears, after which the second (perceived by the other eye) shoots out suddenly in a direction athwart the first. This alternation proceeds for a long time, and produces the curious effect of two luminous blades crossing and recrossing each other.



Another method of bringing out the phenomenon of alternation in a still more striking manner is to look at two different sets of writing, with the two eyes. The resultant effect is a blur, due to superposition, and the inscription cannot be read with the eyes open. But on closing them, the composite image is analysed alternately into its component parts, and thus we are enabled to read better with eyes shut than open.

This period of alternation is modified by age and by the condition of the eye. It is, generally speaking, shorter in youth. I have seen it vary in different individuals from 1" to 10" or more. About 4" is the most usual. With the same individual, again, the period is somewhat modified by previous conditions of rest or activity. Very early in the morning, after sleep, it is at its shortest. I give below a set of readings given by an observer:

Period

8 A.M. 3" 12 noon 4" 3 P.M. 5" 6 P.M. 5.4" 9 " 5.6" 11 " 6.5"

Again, if one eye be cooled and the other warmed, the retinal oscillation in one eye is quicker than in the other. The quicker oscillation overtakes the slower, and we obtain the curious phenomenon of 'visual beats.'

After-images and their revival.—In the experiment with the stereoscope and the design of the cross, the after-images of the cross seen with the eyes closed are at first very distinct—so distinct that any unevenness at the edges of the slanting cuts in the design can be distinctly made out. There can thus be no doubt of the 'objective' nature of the strain impression on the retina, which on the cessation of direct stimulus of light gives rise to after-oscillation with the concomitant visual recurrence. This recurrence may therefore be taken as a proof of the physical strain produced on the retina. The recurrent after-image is very distinct at the beginning and becomes fainter at each repetition; a time comes when it is difficult to tell whether the image seen is the objective after-effect due to strain or merely an effect of 'memory.' In fact there is no line of demarcation between the two, one simply merges into the other. That this 'memory' image is due to objective strain is rendered evident by its recurrence.

In connection with this it is interesting to note that some of the undoubted phenomena of memory are also recurrent. 'Certain sensations for which there is no corresponding process outside the body are generally grouped for convenience under this term [memory]. If the eyes be closed and a picture be called to memory, it will be found that the picture cannot be held, but will repeatedly disappear and appear.'[20]

The visual impressions and their recurrence often persist for a very long time. It usually happens that owing to weariness the recurrent images disappear; but in some instances, long after this disappearance, they will spontaneously reappear at most unexpected moments. In one instance the recurrence was observed in a dream, about three weeks after the original impression was made. In connection with this, the revival of images, on closing the eyes at night, that have been seen during the day, is extremely interesting.

Unconscious visual impression.—While repeating certain experiments on recurrent vision, the above phenomenon became prominent in an unexpected manner. I had been intently looking at a particular window, and obtaining the subsequent after-images by closing the eye; my attention was concentrated on the window, and I saw nothing but the window either as a direct or as an after effect. After this had been repeated a number of times, I found on one occasion, after closing the eye, that, owing to weariness of the particular portion of the retina, I could no longer see the after-image of the window; instead of this I however saw distinctly a circular opening closed with glass panes, and I noticed even the jagged edges of a broken pane. I was not aware of the existence of a circular opening higher up in the wall. The image of this had impressed itself on the retina without my knowledge, and had undoubtedly been producing the recurrent images which remained unnoticed because my principal field of after-vision was filled up and my attention directed towards the recurrent image of the window. When this failed to appear, my field of after-vision was relatively free from distraction, and I could not help seeing what was unnoticed before. It thus appears that, in addition to the images impressed in the retina of which we are conscious, there are many others which are imprinted without our knowledge. We fail to notice them because our attention is directed to something else. But at a subsequent period, when the mind is in a passive state, these impressions may suddenly revive owing to the phenomenon of recurrence. This observation may afford an explanation of some of the phenomena connected with ocular phantoms and hallucinations not traceable to any disease. In these cases the psychical effects produced appear to have no objective cause. Bearing in mind the numerous visual impressions which are being unconsciously made on the retina, it is not at all unlikely that many of these visual phantoms may be due to objective causes.

FOOTNOTES:

[19] As an instance of this I may mention the experiment which I saw on the quick fusion of metals exhibited at the Royal Institution by Sir William Roberts-Austen (1901), where, owing to the glare and the dense fumes, it was impossible to see what happened in the crucible. But I was able to see every detail on closing the eyes. The effects of the smoke, being of less luminescence, cleared away first, and left the after-image of the molten metal growing clearer on the retina.

[20] E. W. Scripture, The New Psychology, p. 101.



CHAPTER XX

GENERAL SURVEY AND CONCLUSION

We have seen that stimulus produces a certain excitatory change in living substances, and that the excitation produced sometimes expresses itself in a visible change of form, as seen in muscle; that in many other cases, however—as in nerve or retina—there is no visible alteration, but the disturbance produced by the stimulus exhibits itself in certain electrical changes, and that whereas the mechanical mode of response is limited in its application, this electrical form is universal.

This irritability of the tissue, as shown in its capacity for response, electrical or mechanical, was found to depend on its physiological activity. Under certain conditions it could be converted from the responsive to an irresponsive state, either temporarily as by anaesthetics, or permanently as by poisons. When thus made permanently irresponsive by any means, the tissue was said to have been killed. We have seen further that from this observed fact—that a tissue when killed passes out of the state of responsiveness into that of irresponsiveness; and from a confusion of 'dead' things with inanimate matter, it has been tacitly assumed that inorganic substances, like dead animal tissues, must necessarily be irresponsive, or incapable of being excited by stimulus—an assumption which has been shown to be gratuitous.

This 'unexplained conception of irritability became the starting-point,' to quote the words of Verworn,[21] 'of vitalism, which in its most complete form asserted a dualism of living and lifeless Nature.... The vitalists soon,' as he goes on to say, 'laid aside, more or less completely, mechanical and chemical explanations of vital phenomena, and introduced, as an explanatory principle, an all-controlling unknown and inscrutable "force hypermecanique." While chemical and physical forces are responsible for all phenomena in lifeless bodies, in living organisms this special force induces and rules all vital actions.

'Later vitalists, however, attempted no analysis of vital force; they employed it in a wholly mystical form as a convenient explanation of all sorts of vital phenomena.... In place of a real explanation a simple phrase such as "vital force" was satisfactory, and signified a mystical force belonging to organisms only. Thus it was easy to "explain" the most complex vital phenomena.'

From this position, with its assumption of the super-physical character of response, it is clear that on the discovery of similar effects amongst inorganic substances, the necessity of theoretically maintaining such dualism in Nature must immediately fall to the ground.

In the previous chapters I have shown that not the fact of response alone, but all those modifications in response which occur under various conditions, take place in plants and metals just as in animal tissues. It may now be well to make a general survey of these phenomena, as exhibited in the three classes of substances.

We have seen that the wave of molecular disturbance in a living animal tissue under stimulus is accompanied by a wave of electrical disturbance; that in certain types of tissue the stimulated is relatively positive to the less disturbed, while in others it is the reverse; that it is essential to the obtaining of electric response to have the contacts leading to the galvanometer unequally affected by excitation; and finally that this is accomplished either (1) by 'injuring' one contact, so that the excitation produced there would be relatively feeble, or (2) by introducing a perfect block between the two contacts, so that the excitation reaches one and not the other.

Further, it has been shown that this characteristic of exhibiting electrical response under stimulus is not confined to animal, but extends also to vegetable tissues. In these the same electrical variations as in nerve and muscle were obtained, by using the method of injury, or that of the block.

Passing to inorganic substances, and using similar experimental arrangements, we have found the same electrical responses evoked in metals under stimulus.

Negative variation.—In all cases, animal, vegetable, and metal, we may obtain response by the method of negative variation, so called, by reducing the excitability of one contact by physical or chemical means. Stimulus causes a transient diminution of the existing current, the variation depending on the intensity of the stimulus (figs. 4, 7, 54).



Relation between stimulus and response.—In all three classes we have found that the intensity of response increases with increasing stimulus. At very high intensities of stimulus, however, there is a tendency of the response to reach a limit (figs. 30, 32, 84). The law that is known as Weber-Fechner's shows a similar characteristic in the relation between stimulus and sensation. And if sensation be a measure of physiological effect we can understand this correspondence of the physiological and sensation curves. We now see further that the physiological effects themselves are ultimately reducible to simple physical phenomena.

Effects of superposition.—In all three types, ineffective stimuli become effective by superposition.

Again, rapidly succeeding stimuli produce a maximum effect, kept balanced by a force of restitution, and continuation of stimulus produces no further effect, in the three cases alike (figs. 17, 18, 86).

Uniform responses.—In the responses of animal, vegetable, and metal alike we meet with a type where the responses are uniform (fig. 112).

Fatigue.—There is, again, another type where fatigue is exhibited.



The explanation hitherto given of fatigue in animal tissues—that it is due to dissimilation or breakdown of tissue, complicated by the presence of fatigue-products, while recovery is due to assimilation, for which material is brought by the blood-supply—has long been seen to be inadequate, since the restorative effect succeeds a short period of rest even in excised bloodless muscle. But that the phenomena of fatigue and recovery were not primarily dependent on dissimilation or assimilation becomes self-evident when we find exactly similar effects produced not only in plants, but also in metals (fig. 113). It has been shown, on the other hand, that these effects are primarily due to cumulative residual strains, and that a brief period of rest, by removing the overstrain, removes also the sign of fatigue.

Staircase effect.—The theory of dissimilation due to stimulus reducing the functional activity below par, and thus causing fatigue, is directly negatived by what is known as the 'staircase' effect, where successive equal stimuli produce increasing response. We saw an exactly similar phenomenon in plants and metals, where successive responses to equal stimuli exhibited an increase, apparently by a gradual removal of molecular sluggishness (fig. 114).



Increased response after continuous stimulation.—An effect somewhat similar, that is to say, an increased response, due to increased molecular mobility, is also shown sometimes after continuous stimulation, not only in animal tissues, but also in metals (fig. 115).

Modified response.—In the case of nerve we saw that the normal response, which is negative, sometimes becomes reversed in sign, i.e. positive, when the specimen is stale. In retina again the normal positive response is converted into negative under the same conditions. Similarly, we found that a plant when withering often shows a positive instead of the usual negative response (fig. 28). On nearing the death-point, also by subjection to extremes of temperature, the same reversal of response is occasionally observed in plants. This reversal of response due to peculiar molecular modification was also seen in metals.



But these modified responses usually become normal when the specimen is subjected to stimulation either strong or long continued (fig. 116).

Diphasic variation.—A diphasic variation is observed in nerve, if the wave of molecular disturbance does not reach the two contacts at the same moment, or if the rate of excitation is not the same at the two points. A similar diphasic variation is also observed in the responses of plants and metals (figs. 26, 68).

Effect of temperature.—In animal tissues response becomes feeble at low temperatures. At an optimum temperature it reaches its greatest amplitude, and, again, beyond a maximum temperature it is very much reduced.

We have observed the same phenomena in plants. In metals too, at high temperatures, the response is very much diminished (figs. 38, 65).

Effect of chemical reagents.—Finally, just as the response of animal tissue is exalted by stimulants, lowered by depressants, and abolished by poisons, so also we have found the response in plants and metals undergoing similar exaltation, depression, or abolition.

We have seen that the criterion by which vital response is differentiated is its abolition by the action of certain reagents—the so-called poisons. We find, however, that 'poisons' also abolish the responses in plants and metals (fig. 117). Just as animal tissues pass from a state of responsiveness while living to a state of irresponsiveness when killed by poisons, so also we find metals transformed from a responsive to an irresponsive condition by the action of similar 'poisonous' reagents.

The parallel is the more striking since it has long been known with regard to animal tissues that the same drug, administered in large or small doses, might have opposite effects, and in preceding chapters we have seen that the same statement holds good of plants and metals also.

Stimulus of light.—Even the responses of such a highly specialised organ as the retina are strictly paralleled by inorganic responses. We have seen how the stimulus of light evokes in the artificial retina responses which coincide in all their detail with those produced in the real retina. This was seen in ineffective stimuli becoming effective after repetition, in the relation between stimulus and response, and in the effects produced by temperature; also in the phenomenon of after-oscillation. These similarities went even further, the very abnormalities of retinal response finding their reflection in the inorganic.



Thus living response in all its diverse manifestations is found to be only a repetition of responses seen in the inorganic. There is in it no element of mystery or caprice, such as we must admit to be applied in the assumption of a hypermechanical vital force, acting in contradiction or defiance of those physical laws that govern the world of matter. Nowhere in the entire range of these response-phenomena—inclusive as that is of metals, plants, and animals—do we detect any breach of continuity. In the study of processes apparently so complex as those of irritability, we must, of course, expect to be confronted with many difficulties. But if these are to be overcome, they, like others, must be faced, and their investigation patiently pursued, without the postulation of special forces whose convenient property it is to meet all emergencies in virtue of their vagueness. If, at least, we are ever to understand the intricate mechanism of the animal machine, it will be granted that we must cease to evade the problems it presents by the use of mere phrases which really explain nothing.

We have seen that amongst the phenomena of response, there is no necessity for the assumption of vital force. They are, on the contrary, physico-chemical phenomena, susceptible of a physical inquiry as definite as any other in inorganic regions.

Physiologists have taught us to read in the response-curves a history of the influence of various external agencies and conditions on the phenomenon of life. By these means we are able to trace the gradual diminution of responsiveness by fatigue, by extremes of heat and cold, its exaltation by stimulants, the arrest of the life-process by poison.

The investigations which have just been described may possibly carry us one step further, proving to us that these things are determined, not by the play of an unknowable and arbitrary vital force, but by the working of laws that know no change, acting equally and uniformly throughout the organic and the inorganic worlds.

FOOTNOTES:

[21] Verworn, General Physiology, p. 18.



INDEX

Action current in metal, 88 in nerve, 8 in plant, 19

After-images and their revival, 177

After-oscillation in photo-sensitive cell, 159, 163

Anaesthetics, effect on response in nerve, 72 in plant, 30, 73, 74, 75

Annealing, effect on response in metal, 101, 138

Binocular alternation of vision, 175

Block method, advantages of, 28, 77 for obtaining response in metal, 82 in plant, 28

Chloral, effect on plant response, 75

Chloroform, effect on nerve response, 72 plant response, 74

Compensator, 22

Current of injury in nerve, 7

Curves, characteristics of response, 3

Death-point, determination of, in plants, 61, 63

Depressants, effect on inorganic response, 142

Depression, response by relative, 87

Dewar on retinal current, 149

Diphasic variation in metal, 113, 114, 115, 116, 188 in nerve, 188 in plant, 46, 188

Dose, effect on inorganic response, 89, 146, 189 plant response, 79, 189

Electrical recorder, 11

Electrical response. See Response, electrical

Electric tapper, 24

Exaltation, response by relative, 89

Fatigue, absence of, under certain conditions, in metal, 120 in muscle, 39 in plant, 39 apparent, with increased frequency of stimulation, in metal, 120 in muscle, 40 in plant, 40 diminution of response under strong stimulus due to, in plant, 57 in metal, 118, 119, 185 in muscle, 118, 185 in plant, 20, 185 due to overstrain, 41 rapid, under continuous stimulation in metal, 121, 130 in muscle, 42, 130 in plant, 42, 130 removal of, by rest in plant, 43 theory of, in muscle, 38, 185

Holmgren on retinal current, 149

Hysteresis, 137

Injury, current of, in nerve, 7

Inorganic response. See Metal, electrical response in

Kuhne on retinal current, 149

Kunkel on electrical changes by injury or flexion in plant, 14, 70

Light, after-effect of short exposure to, on photo-sensitive cell, 171 on retina, 171 decline and reversal of response under continuous, in photo-sensitive cell, 166 in retina 166 effect of temperature on response of photo-sensitive cell produced by, 158 retinal response produced by, 158 relation between intensity and response to, in photo-sensitive cell, 161, 162 in retina, 162 response to, after-oscillation in photo-sensitive cell, 159, 163 effect of increasing length of exposure in photo-sensitive cell, 159 in retina, 160 in frog's retina, 150, 151, 156, 164, 166 in photo-sensitive cell, 152, 153, 154, 155, 157, 165, 166

McKendrick on retinal response, 149

Mechanical recorder, 3 response, 1 stimulus by electric tapper, 24 by spring-tapper, 23 by vibrator, 24 conditions of maintaining uniformity of, 26 means of graduating intensity of, 22, 24, 96

Metal, electric response in, abnormal, 125 abolition of, by 'poison,' 143 additive effect of superposition of stimulus on, 135 annealing, effect of, on, 101 by method of block, 82, 92 negative variation, 87, 183 depressants, effect of, on, 142 diphasic, 113, 114, 115, 116, 188 enhancement of, after continuous stimulation, 127, 128, 186 fatigue, 118, 119, 120, 121, 185. See also Fatigue maximum effect due to superposition of stimuli, 136 modified, 129 'molecular arrest,' effect of, by 'poison' on, 145 molecular friction, effect of, on, 108, 109 prolongation of recovery by overstrain, 106 by 'poison,' 145 relation between, and stimulus, 134, 135 staircase effect, 122, 186 stimulant, effect of, on, 141 temperature, effect of, on, 111 uniform, 102, 184

Minchin on photo-electric cell, 165

Molecular 'arrest' in metals by 'poison,' 145 friction, 108, 109 model, 107 voltaic cell, 99

Munck on electric response in sensitive plants, 14

Muscle, fatigue in, 38, 39, 40, 42. See also Fatigue prolongation of recovery by 'poison' in, 144 relation between stimulus and response in, 52 staircase effect in, 122 stimulus, effect of superposition of, on, 36

Myograph, 2

Negative variation, response by method of, in metal, 87, 183 in nerve, 9, 183 in plant, 18, 183

Nerve, current of injury in, 7 injured and uninjured contacts corresponding to Cu and Zn in voltaic couple, 8 response in, abnormal, when stale, 124, 187 abolition of, by 'poison,' 139, 189 anaesthetics, effect of, on, 72 by method of negative variation, 9 current of action of, 8 enhancement of, after continuous stimulation, 127 modified, 128 relation between, and stimulus, 52 reversed when stale, 11 uniform, 184

Nomenclature, anomalies of present, 9, 85

Photographic recorder, 11, 22

Plant chamber, 64 electrical response in, abnormal, when stale or dying, 48, 187 abolition of, by high temperature, 32, 64 additive effect of stimulus on, 37 anaesthetics, effect of, on, 30, 73, 74, 75 by method of block, 28 of negative variation, 18, 183 diphasic, 46 fatigue, 20, 39, 40, 41, 42, 43, 57, 185. See also Fatigue physiological character, 30 'poison,' effect of, on, 30, 32, 78, 79 relation between, and stimulus, 52, 53, 54 staircase effect, 37, 185 stimulus, effect of single, on, 35 effect of superposition of, on, 35 temperature, effect of, on, 32, 59-69 uniform, 36, 184 radial E.M. response in, 49

Poison, effect of, on response in metal, 143, 189 in nerve, 139, 189 in plant, 30, 32, 78, 79, 189 'molecular arrest' in metal by, 145 prolongation of recovery by action of, in metal, 145 in muscle, 144

Record, simultaneous mechanical and electrical, of response, 13

Recorder, electrical, 11 mechanical, 3 photographic, 11, 22 response, 19

Response-curve, characteristics of, 3 electrical, abnormal, in metal, 123, 125 in stale nerve, 11, 123 in stale or dying plant, 48, 187 in stale retina, 11, 164 converted into normal after strong or continuous stimulation in metal, 125, 187 in nerve, 124, 187 in plant, 48 abolition of, by high temperature in plant, 32, 64 by 'poison,' in metal, 143, 189 in nerve, 139, 189 in plant, 30, 32, 78, 79, 189 additive effect of stimulus on, in metal, 135 in plant, 37 anaesthetics, effect of, on, in nerve, 72 in plant, 30, 73, 74, 75 annealing, effect of, on, in metal, 101, 138 by method of block, 28, 82, 92 by negative variation, 9, 18, 87, 183 by relative depression, 87 by relative exaltation, 89 conditions for obtaining, 6, 86, 87 continuous transformation from positive to negative in metal, 115 decline and reversal of, under continuous light in photo-sensitive cell, 166 decline and reversal of, under continuous light in retina, 166 depressants, effect of, on inorganic, 142 diminution of. See Fatigue diphasic in metal, 113, 114, 115, 116, 188 in nerve, 188 in plant, 46, 188 dose, effect of, on inorganic, 89, 146, 189 on, in plant, 79, 189 enhancement of, after continuous stimulation in metal, 127, 128, 186 enhancement of, after continuous stimulation in nerve, 127, 186 maximum effect due to superposition of stimulus, 35, 136 measure of physiological activity, 13 molecular friction, effect of, on, 108, 109 modification, effect of, on, 11, 48, 123, 125, 129, 164, 187 physiological character of, in plant, 30 positive and negative, 11 prolongation of recovery in, by 'poison' in metal, 145 prolongation of recovery in, by 'poison' in muscle, 144 prolongation of recovery in, from overstrain, 106 relation between, and stimulus in metal, 134, 135 in muscle, 52 in nerve, 52 in plant, 52, 53, 54 in real and artificial retinae, 162 staircase effect, in metal, 122, 186 in plant, 37, 186 stimulant, effect of, on, in metal, 141 temperature, effect of, on. See Temperature threshold of, 135 to light. See Light uniform in metal, 102, 184 in nerve, 184 in plant, 36, 184 universal applicability of, 12 mechanical, 1 retinal. See Light simultaneous mechanical and electrical record of, 13

Retina. See Light

Sanderson, Burdon-, on electrical response in sensitive plants, 14

Spring-tapper, mechanical stimulus by, 23

Staircase effect in metal, 122, 186 in muscle, 122, 186 in plant, 37, 186

Steiner on retinal response, 149

Stimuli, maximum effect due to superposition of, in metal, 136 in muscle, 36 in plant, 36

Stimulus, advantages of vibrational, 25 and response, relation between, in metal, 134, 135 in muscle, 52 in nerve, 52 in plant, 52, 53, 54 in real and artificial retinae, 162 effect of different kinds of, 2 mechanical, by spring-tapper, 24 conditions for maintaining uniformity of, 26 means of graduating intensity of, 22, 96 vibrational, 24, 25, 26

Temperature, death-points in plants, 61, 63 effect of, on response in metal, 111 in photo-sensitive cell, 158 in plants, 32, 60-69 in retina, 158 increased sensitiveness in plant due to variation of, 66, 67

Vibrational stimulus, 24, 25, 26

Vision, binocular alternation of, 175 effect of various conditions on the period of binocular alternation of, 177

Visual images, revival of, 177 impression, unconscious, 178 impulse, chemical theory of, 148 electrical theory of, 149 phantoms, 179 recurrence, 174

Vital force, 13

Vitalism, 182

Waller on enhancement of nerve-response after continuous stimulation, 127 on relation between stimulus and response in muscle, nerve, and retina, 52, 162 on retinal response, 150, 156, 165 on reversal of response in stale nerve and retina, 11, 124, 164 on transformation from abnormal to normal response in nerve after continuous stimulation, 124



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Transcriber's note: The following printer errors have been corrected: Diaphasic changed to Diphasic (fig. 26 caption) Dash added (section on Continuous Transformation) Blurr changed to blur (after fig. 111) In creased changed to increased (after fig. 114) The inconsistent hyphenation of "break-down", "electro-motive" and "vibration-head" is as in the original.

THE END