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 



911 



