ORGANS OF PERCEPTION. 183 
purpose, the humours of the eye are so adjusted, in their 
form, density and refractive power, as to prevent any dis- 
persion or decomposition of the rays. They thus act in a 
similar manner to the compound object-glasses of an achro- 
matic telescope. 
As animals reside in different media, it is obvious that 
the eyes of each must possess different refractive powers. 
In the land animals the cornea is usually convex, and the 
aqueous humour abundant; while in aquatic animals 
the former is flat, and the latter in small quantity. In 
land animals, the aqueous humour possesses great. power 
of refracting rays, passing to it through air, aided likewise 
by the convexity of its surface. But its refractive power in 
water would be comparatively weak. 'This defect, however, 
is supplied by the spherical form and great refractive power 
of the lens, as may be seen in whales, diving-birds, and fishes. 
When we look-at the image of an object in the focus 
of a convex lens, or in that of the natural eye of a re- 
cently slaughtered bullock, prepared by removing the coats 
on its posterior side, and thrown upon white paper, we ob- 
serve a picture formed, but in a reversed position,—the rays 
of light emanating from the upper part.of the object. form- 
ing the lower part of the image, and those from the right 
proceeding to the left. If the retina, in the living eye, be 
considered. as occupying the place of the white paper in 
these experiments, it must follow, that the image of any 
object thus painted on the retina will be inverted. How 
comes it to pass, therefore, that we see every object in its 
natural upright position ? All this difficulty originates in a 
misconception of the nature of the retina, and the impres- 
sions which it receives. There is no white screen in the 
eye, on which the image of an object can be painted. The 
retina is translucent, and the choroid behind it 1s black. 
The retina is not, therefore, acted upon by the reflected 
