LECT. XX. THE STEREOSCOPE. 381 



tance, if it be properly illuminated in its different parts, 

 gives us the perfect image of the original, so that the illu- 

 sion is complete ; of this we have an example in the diorama. 

 But this is no longer the case when the object is at a very 

 short distance from the eye. We are indebted to Wheat- 

 stone for a series of extremely ingenious experiments upon 

 this subject. When a solid body, a cube for instance, is 

 very near the eyes, its projection on the retina of each eye 

 forms there two different images ; they resemble each other 

 so slightly, that if we supposed them drawn, we could 

 scarcely, by looking at them, recognise that they belonged 

 to the same body. Notwithstanding this difference, we see 

 a single object: we must then conclude, from these facts, 

 that the perception in relief may be produced by the simul- 

 taneous impression of two images formed in each eye ; in 

 other words, to see objects as they are, becomes an illusion. 

 Notwithstanding the observations of Wheatstone, we must, 

 however, assume, that a single eye is capable of estimating 

 the solidity of bodies, as is seen daily in persons who have 

 lost one. Experience, custom, and the other senses, assist 

 in correcting this defect. 



The Stereoscope. By looking at the same time at the 

 images of two drawings, obtained by copying the two pro- 

 jections of a solid body upon the retinse of the two eyes, 

 Wheatstone succeeded in producing the same sensation as 

 that which would have been produced by the solid body. 

 When the observation is made in such a way that the images 

 of the two drawings are formed in the same manner, and 

 upon the same points of the retina which the two projections 

 of the solid occupy, the illusion is complete, and it is im- 

 possible to believe that we have before the eyes only paint- 

 ings made upon a plane. 



