APPARENTLY SEEN THROUGH THE HAND 157 



in the left hand, look through it with the left eye, the right 

 eye also being kept open. Then bring the right hand into 

 the position shown in the engraving, Fig. 33, the edge op- 

 posite the thumb being about in line with the right-hand 

 side of the tube. Or the right hand may rest against the 

 right-hand side of the tube, near the end farthest from the 

 eye. This cuts off entirely the view of the object by the 

 right eye, yet strange to say the object will still remain 

 apparently visible to both eyes through a hole in the hand, 

 as shown by the dotted lines in the engraving ! In other 

 words, it will appear to us as if there was actually a hole 

 through the hand, the object being seen through that hole. 

 The result is startlingly realistic, and forms one of the 

 simplest and most interesting experiments known. 



This singular optical illusion is evidently due to the sym- 

 pathy which exists between the two eyes, from our habit of 

 blending the images formed in both eyes so as to give a 

 single image, 



