Fig. 111.—Stereoscopic Design
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.
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 | Period | ||||
|---|---|---|---|---|---|
| 8 | a.m. | 3″ | 6 | p.m. | 5·4″ |
| 12 | noon | 4″ | 9 | " | 5·6″ |
| 3 | p.m. | 5″ | 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]