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.
[Illustration: 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 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 retinæ 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.
[Illustration: 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 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.