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 subst.i.tute. 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.

[Ill.u.s.tration: FIG. 110--RESPONSE-CURVES OF THE SENSITIVE SILVER CELL Showing greater persistence of after-effect when the stimulus is strong.

(_a_) Short exposure of 2" to light of intensity 1; (_b_) short exposure of 2" to light nine times as strong.]

#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 pa.s.ses 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 gla.s.s 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 r.e.t.a.r.dation 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.

[Ill.u.s.tration: FIG. 111.--STEREOSCOPIC DESIGN]

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 a.n.a.lysed 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. 54"

9 " 56"

11 " 65"

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 repet.i.tion; 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 gla.s.s 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 princ.i.p.al 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 pa.s.sive 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 Inst.i.tution 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 pa.s.ses out of the state of responsiveness into that of irresponsiveness; and from a confusion of 'dead' things with inanimate matter, it has been tacitly a.s.sumed that inorganic substances, like dead animal tissues, must necessarily be irresponsive, or incapable of being excited by stimulus--an a.s.sumption 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 a.s.serted 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 a.n.a.lysis 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 a.s.sumption 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 cla.s.ses 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.