ANJS 01' I'lvUCErTION. 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- 

 siom jch 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 is black. 

 The retina is not, therefore, acted upon by the reflected 



