FORMATION OF IMAGE ON THE RETINA. 423 
in a focus on the retina. As every point is thus repre¬ 
sented in its proper position relatively to others (except that 
those which were above are now below, and vice versd), a 
complete inverted image or picture of the object is formed 
upon the retina. This is shown in fig. 210; where, for the 
sake of convenience, three rays only are represented as issuing 
from the centre and the two extremities of an object a b. These 
rays cross each other at h , in the middle of the eye ; so that 
those from a being brought to a focus at c , and those from 
b at d, and all the other rays being refracted in the same 
manner, a complete inverted picture of the object is formed at 
the back of the eye. 
54:4c. That this is really the case, may not only be inferred, 
but proved. If the eye of a Eabbit be removed from its 
socket, and cleansed of the muscles, fat, &c., adherent to its 
back part, and a candle be then brought in front of it, the 
transparency of the sclerotic coat will allow the image of the 
candle that is formed upon the retina to be distinctly seen. 
Or, if we take the eye of a Sheep or an Ox, and after cleans¬ 
ing it in the same manner, we cut out from the back of it- a 
portion of the sclerotic and choroid coats, covering the part 
of the retina thus left bare with a piece of tissue-paper (for 
the purpose of keeping-in the vitreous humor, without 
interrupting our view of the image), a distinct but inverted 
miniature picture of all the objects in front of the eye will 
be seen at its back. It is necessary in these experiments 
that the eyes should be taken from animals recently killed; 
as the cornea and humors soon lose their transparency, and 
the distinctness of the picture is consequently impaired. 
54:5. The black pigment, which is situated immediately 
behind the retina,—that is, in contact with its external 
surface,—is destined to absorb the rays of light immediately 
that they have passed through the retina; so as to prevent 
them from being reflected from one part of the interior of 
the globe of the eye to another, which would cause a great 
confusion and indistinctness in the picture. Hence it is that 
in those individuals (both among Man and the lower animals) 
in whose eyes this pigment is deficient, vision is extremely 
imperfect. The eyes of such individuals (termed Albinos) 
derive from the absence of pigment a very peculiar appear¬ 
ance. The iris does not possess its ordinary colour; but, 
