Page:Popular Science Monthly Volume 92.djvu/927

 Looking Through Your Hand

��An optical illusion and at the same time a valuable test

��IN a recent issue of "La Nature" H. Volta speaks interestingly of an instrument which he calls "The il- lusion of the hole through the hand" and incidentally points out the value of that experiment as a clinical test for cross-eyes and other defects of vision.

Place before one eye, the right one, for instance, a roll of paper or a pasteboard tube; then hold the left hand, open, to the left of the tube, at a distance of about ten inches from the left eye. The right eye sees what is framed in by the tube; the left eye sees the hand. In the brain the images of the two eyes are fused to-

���gether; it seems as if the left hand were perforated by a hole through which can be seen the objects included in the field of the tube. What is the explana- tion of this curious phenomenon?

When we look at an object, each eye sees it, but the impressions of the retinas are transmitted in such a way as to register but a single image in the brain. That is what the physicians express by saving that the rays emanating from the same object strike identical points of our retinas; the optical nerves leading from these points are combined so as to con- nect with a single brain cell. If some trouble affects on^' of the eyes, the points of the two retinas which have received impressions are no longer symmetrical; the two images cannot be fused into a

��single brain perception; we see double, a condition which is known scientifically as diplopia.

The explanation is quite simple, but, and this is the interesting feature of this experiment, one may deduct from it the most interesting clinical indications, which make it possible to combat from the start the cross-eyedness with which chil- dren are troubled so frequently and which parents too often have a tendency to neglect.

Cross - eyedness de- velops in children gradually — so grad- ually, in fact, that the eyes become ac- customed to the de- fect and do not be- tray its existence by seeing double. The functions of the eye with the less perfect vision are simply neu- tralized. When the good eye is closed, the poor eye sees; but when the good eye is opened again the im- ages conveyed by the poor eye are again dis- regarded by the brain. Although there is no double vision, bin- ocular vision is no longer possible.

By means of the experiment described it is possible to ascertain whether correct binocular vision is possible to the person tested. With normal vision the apparent hole should be seen in the middle of the hand. If the hand or the view in the field of the tube is not seen, one of the eyes does not see a. all, or its impressions are neutralized. If cross-eyedness exists, the hole will appear to the right or the left, above or below the hand. The dis- tance at which the left hand has to be held from the end of the tube before the hole most nearly approaches its center differs according to the degree of cross- eyedness, and will furnish the eye special- ist valuable information.

��With normal vision a per- son trying this experiment should see an apparent opening in the middle of the hand, as shown in the picture on the left

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