192 PHILOSOPHY 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 A/binoes. Their eyes 
are tender, and impatient of hght +. 
Many animals can only see an object with one eye ata 
time. But in other animals, as man, both eyes 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 
dees not exist. Were this not the case, the compound 
eyes of insects would exhibit objects multiplied to an extent 

* Phil. Trans. 1799, p. 1. 
+ Buumensach, in reference to this subject, offers the following inter- 
esting observations. ‘ It is well known that this pigment is entirely, or for 
the greatest part, deficient in the eye of the albinoes or chacrelas; which strange 
variety occurs, not unfrequently, in the human race, and in several other 
mammalia and birds. J know, however, no instance of an albino among 
cold-blooded animals. This anomalous deficiency is always congenital; and 
is connected with a want of the colouring principle of the skin, and of the 
hair and feathers. It is hereditary in some mammalia, so as to form a con- 
stant breed of white aniamls, viz. in the rabbit, mouse and horse, (which 
latter are those called glass-eyed.) I cannot believe that any whole species 
of warm-blooded animals should originally want this pigment; and, 
therefore, I consider the ferret, (Mustela furo,) to have descended from the 
polecat (M. putorius.”) Comparative Anatomy, Trans. Note to p. 363. 
