280 THE APPLICATIONS OF PHYSICAL FORCES. [BOOK in. 



This first point proved What is the result when we examine an 

 object in relief with one eye ? This may be tested in the simplest 

 manner. Let us take for example a cube die (Fig. 208), or a quadran- 

 gular pyramid. Let us place them both in a vertical plane passing 

 between the two eyes, and look a,t each with both eyes together : the 

 two figures A and B will represent the two objects seen in that way. 

 If we close the left eye the aspect will change. The right lateral 

 surface of the die A' will be more visible, while the left will have 

 disappeared ; the lateral surfaces of the pyramid B' will also appear 

 of unequal size. If we now close the right eye, an opposite effect 

 will be produced as shown at A" and B". We may make a thousand 



FIG. 208. Difference between monocular and binocular vision. 



similar experiments on near or distant objects. We shall find that 

 the sight, with the right eye alone, discovers parts which remain 

 hidden when we use only the left eye. From this we conclude that 

 a different picture of the same object is painted on each retina, right 

 and left, so that we might expect as a result of binocular vision a 

 double, picture. But experience proves that this is not the case, and 

 that these two pictures are so superposed as only to give one distinct 

 impression in W 7 hich the different parts of the two pictures are united. 

 Complete or normal vision envelops, so to speak, objects in relief, 

 and the more so the nearer thev are. 



