876 



THE SENSE OF SIGHT 



The Stereoscope. The purpose of this instrument is to fuse non- 

 corresponding images so that they may give rise to a single visual con- 

 cept possessing solidity. Wheatstone 1 accomplished this end by 

 means of mirrors placed at certain angles to one another, and Brewster 2 

 by means of two prisms. It has been stated above that the images 

 of a solid object in binocular vision, are somewhat different in the 

 two eyes, but since they are formed upon corresponding points of the 

 retinas, they produce a perfectly harmonious relief. 



The stereoscope serves to give solidity to pictures of objects which 

 would otherwise present only two dimensions, namely those of height 

 A and breadth. This fusion of two pic- 



tures is effected by permitting each eye 

 / \ \/ to regard its own field through a curved 



/' \ /, prism composed of two half lenses with 



convex surfaces, the inner margins of 

 which are directed inward. A vertical 

 screen is adjusted between the two 

 lenses in such a way that the sight of 

 the left eye is entirely cut off from that 

 of the right eye (Fig. 475). While the 

 prisms themselves tend to magnify the 

 pictures, stereoscopic views are usually 

 taken with the help of two lenses which 

 are separated by a distance somewhat 

 greater than the interocular. Conse- 

 quently, the solidity is in reality some- 



STER STEREOSCOPE, what exaggerated. It has been pointed 



P and P', the prisms, a, b, and out above that two identical pictures 



a, 0, the left- and right-eyed pictures, carmo t give a Sensation of relief, be- 

 respectively, o , p, being a point in the , . , , . . . 



foreground and a, a, a point in the cause only the psychic comparison of 

 background. The eyes are converged two slightly dissimilar images can lead 



and focused separately for each point to & p erception o f so lidity. In Other 

 as in viewing naturally an object of 11 11 



three dimensions. (Landois.) words> we have learned by experience 



that only those objects can give rise to 



this impression which possess solidity. Upon this basis rests the 

 psychic interpretation of stereoscopic pictures. 



Optical Illusions. It appears, therefore, that seeing is essentially 

 a process of reasoning in accordance with past experience. This 

 point is clearly proved by the visual sensations of blind persons whose 

 sight has been restored in later years by the sudden relief of the condi- 

 tion causing the blindness. Although these persons have gradually 

 formed an idea regarding the shape and size of objects by 

 means of the muscle-sense, they are then quite unable to tell 

 ''which is which," and several days of repeated comparison are required 



1 Phil, transactions, 1838. 



2 Edinbourgh Phil. Transactions, 1843; also: Rollman, Poggend. Annalen, 

 1853. 



