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PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 



portion to their distance, and we judge of distance in 

 this way. 



2. Mathematical Perspective. — The angular diameter 

 of objects, and therefore the size of the retinal image, is 

 mathematically proportioned to the distance. Therefore 

 objects seem small in proportion to their distance. Par- 

 allel lines, like railway tracks, converge, and houses on 

 the two sides of a street converge and grow smaller 

 with distance. We judge of distance quite accurately in 

 this way. 



3. Binocular Perspective. — This, as already explained, 

 is a judgment of distance by running the point of sight 

 back and forth, successively uniting double images by 

 greater and less convergence, and thus gauging space. 



4. Focal Perspective. — When with one eye we look at 

 a very near object, farther ones are dim, and vice versa. 

 We are aware of voluntary effort of accommodation 

 for distinct vision of near objects, and judge of relative 

 distance in this way also. 



Distance at which these Operate. — Now, of these 

 four kinds, the focal operates for only about twenty 

 feet. Beyond this the accommodation is a vanishing 

 quantity. The binocular perspective operates for about 

 one quarter to one half mile. Beyond this it, too, be- 

 comes a vanishing quantity. The other two operate 

 without limit. 



The painter can imitate the first and second, and much 

 of his art consists in skillfully introducing an appear- 

 ance of distance by dimming and bluing and making 

 smaller the objects in the background of his picture. 

 The other two he can not imitate. The lack of focal 

 perspective is, however, of little importance, because 

 landscape pictures are usually viewed at a consider- 

 able distance. But the lack of binocular perspective 

 seriously interferes with the illusion which he seeks to 



