192 PHILOSOPHV OF ZOOLOGY. 



probable, that both surfaces of the retina are equally 

 adapted for receiving impressions of external objects ; and, 

 judging from analogy, it is probable, that the rays in their 

 passage inwards alone produce the image *. 



In some individuals of certain species of quadrupeds and 

 birds, the mucous pigment is entirely deficient ; so that 

 the choroid coat is visible through the iris. This deficien- 

 cy is always congenital, and is connected with a defect of 

 the secreting organs of the colouring matter of the hair and 

 feathers. Such animals are called Albino?*. Their eyes 

 are tender, and impatient of light J ~. 



Many animals can onlv ste an object with one eye at a 

 tine. But in other animals, as man. both eves may be di- 

 rected at once to the same object, so as to produce an image 

 in the retina of each eye- Still, however, we see objects 

 simple ; and this single vision has, by some, been ascribed 

 to habit. It is, however, probable, that vision is always 

 single, when the images fall on precisely the corresponding 

 points of both retinae, and only double where this, condition 

 does not exist Were this not the case, the compound 

 eyes of insects would exhibit objects multiplied to an extent 



Phi Trans. 1799, p. 1. 



t BLrxnrBACH, in nftnim to this subject, oilers the following inter- 

 It b weO known that this pigment is entirety, or for 

 part, deficient in the eye of the 



is the human race, 



I with a want of the colouring principle of the skin, and of the 



att breed of white amamls,Tiz. in the rabbit, moose and hone, (which 

 ! are those caDed gfao^edL) I cam* befiere that any whole species 

 of wana-blooded animal, should originally want this pigment; and, 

 therefore, Iconaider the ferret, (Mustela faro,) to hare descended from the 

 : fM. pourin*.^ r ~*~ A*m*my, Trww. Ne to p, 363. 



