THE STEREOSCOPE. 



537 



rays of light so that objects seen through the prism 

 appear to be nearer to the refracting edge ; the prism /? t 

 therefore refracts the ray a p l in the direction p 4 A, as 

 if it proceeded from c. The prism p 2 refracts the ray 

 bp. 2 so that to the eye at B it also appears to proceed 

 from c. The effect of this is provided that the two 

 pictures a and b are drawn just as a body at c would 

 appear to the eyes at A and B if the prisms were not 

 there that the object really appears to be at c. 

 \ as the points a and b combine to form the point c, 

 so d and e unite to form the point /, g and h to form 

 the point i. 

 In most stereoscopes the prisms have not flat but convex 



Fre, 294 (| 



faces, and therefore, in addition to causing the lateral 

 displacement, act like convex lenses, and magnify the 



