Yet in spite of all these drawbacks our eyes do us excellent service, and provided that they are free from actual malformation and have not suffered from injury or disease, we do not often find fault with them. This, however, is not because they are as good as they might be, but because with incessant practice we have acquired a very high degree of skill in their use. If anything is more remarkable than the ease and certainty with which we have learnt to interpret ocular indications, when they are in some sort of conformity with external objects, it is the pertinacity with which we refuse to be misled when our eyes are doing their best to deceive us. In our earliest years we began to find out that we must not believe all we saw; experience gradually taught us that on certain points and under certain circumstances the indications of our organs of vision were uniformly meaningless or fallacious, and we soon discovered that it would save us trouble and add to the comfort of life if we cultivated a habit of completely ignoring all such visual sensations as were of no practical value. In this most of us have been remarkably successful; so much so, that if, from motives of curiosity, or for the sake of scientific experiment, we wish to direct our attention to the sensations in question, and to see things as they actually appear, we can only do so with the greatest difficulty; sometimes, indeed, not at all, unless with the assistance of some specially contrived artifice.
In the present chapter it is proposed to discuss a few of the less familiar vagaries of the visual organs, and to show how they may be demonstrated. Some of the experiments may, it is to be feared, be found rather difficult; success will depend mainly upon the experimentalist’s ability to lay aside habit and prejudice, and give close attention to his visual sensations; but it is hardly to be expected that an unskilled person will at the first attempt observe all the phenomena which will be referred to.
Among the most annoying of the eccentricities which characterise the sense of vision is that known as the persistence of impressions. The sensation of sight which is produced by an illuminated object does not cease at the moment when the exciting cause is removed or changed in position; it continues for a period which is generally said to be about a tenth of a second, but may sometimes be much more or less. It is for this reason that we cannot see the details of anything which is in rapid motion, but only an indistinct blur, resulting from the confusion of successive impressions. If a cardboard disk, which is painted in conspicuous black and white sectors is caused to rotate at a sufficiently high speed, the divisions are completely lost sight of, and the whole surface appears to be of a uniformly grey hue. But if the rapidly rotating disk is illuminated by a properly timed series of electric flashes, it looks as if it were at rest, and in spite of the intermittent nature of the light, the black and white sectors can be seen quite continuously, though as a matter of fact the intervals of darkness are very much longer than those of illumination. Persistent impressions of this kind are often spoken of as positive after-images.
There is a very remarkable phenomenon accompanying the formation of positive after-images, especially those following brief illumination, which seems, until comparatively recent times, to have entirely escaped the notice of the most acute observers. It was first observed accidentally by Professor C. A. Young, when he was experimenting with a large electrical machine which had been newly acquired for his laboratory. He noticed that when a powerful Leyden jar discharge took place in a darkened room, any conspicuous object was seen twice at least, with an interval of a trifle less than a quarter of a second, the first time vividly, the second time faintly. Often it was seen a third time, and sometimes, but only with great difficulty, even a fourth time. He gave to this phenomenon the name of recurrent vision; it may perhaps be more appropriately denominated the Young effect.
By means of the powerful machine presented to the Royal Institution by Mr. Wimshurst, used in conjunction with a battery of Leyden jars, the Young effect has been successfully shown to a large assembly. But it is quite easy to demonstrate it on a small scale with any influence machine which will give a spark about an inch long. One of the terminals of the machine should be connected by a wire with the inner coating of a half-pint Leyden jar, the other with the outer coating, and the discharging balls should be set a quarter of an inch apart. The observer’s eyes must be shielded from the direct light of the spark by any convenient screen, such as a large book set on end. The best object for the experiment is a sheet of white paper, placed in an upright position a few inches away from the terminals of the machine and exposed to the full light of the discharge.
The room being darkened, let the machine be worked slowly, while the eyes are turned towards the white paper. This will be seen for a moment when the spark passes, and, after a dark interval of about one-fifth of a second, it will make another brief appearance. After a further short interval of darkness, a second recurrent image will often be seen. It may be remarked that the effect is most striking when the eyes are not directed exactly upon the white paper, but above or on one side of it; the proper distance of the paper from the spark-gap should be found by trial.
Under favourable conditions I have observed as many as six or seven reappearances of an object which was illuminated by a single discharge. These followed one another at the usual rate—about five in a second—and produced a twinkling or quivering effect, closely resembling that attending a flash of lightning which is not directly seen. There can indeed be little doubt that the proverbial quiver of the lightning-flash is in many cases merely an effect of recurrent vision, though sometimes, of course, as has been shown by photographs, the discharge is really multiple.
Some years ago I called attention to a very different method of exhibiting a recurrent image. The apparatus used for the purpose consists of a vacuum tube mounted in the usual way upon a horizontal axis capable of rotation. When the tube is illuminated by a rapid succession of discharges from an induction coil, and is made to rotate very slowly by clockwork (turning once in every two or three seconds), a very curious phenomenon may be noticed. At a distance of a few degrees behind the tube and separated from it by an interval of perfect darkness, comes a ghost. This ghost is in form an exact reproduction of the tube; it is very clearly defined, and though its apparent luminosity is somewhat feeble, it can in most cases be seen without difficulty. The varied colours of the original are, however, absent, the whole of the phantom tube being of a uniform bluish or violet tint. If the rotation is suddenly stopped the ghost still moves steadily on until it reaches the luminous tube, with which it coalesces and so disappears. (See [Fig. 36], where the recurrent image is represented by dotted lines.)
