ANIMAL MECHANISM. 



the rays of light from external objects which reach it, are 

 united on it, through the transparent part of the eye, and 

 it again reflects back the whole united rays towards 

 the lens. This latter unites them into a single cone, 

 which has the occular axis as its axis, and its point is 

 directed outwards. The very convergent rays of this 

 cone become more divergent by their passage from the 

 lens into the aqueous fluid, and from this into air or 

 water. Finally, the apex of this cone falls into the point 

 of most distinct vision ; for in this point is situated the 

 focus of all the rays that reach from the interior of the 

 eye to the posterior surface of the lens. The cone is 

 complete when the tapetum is spread over the whole of 

 the choroid ; but the upper half of it is wanting, when 

 it occupies only the upper hemisphere of the coat. The 

 tapetum is confined to the upper half of the choroid in all 

 animals, whose residence and manner of life are of such 

 a nature, that the under half of the retina is immediately 

 struck by bright daylight, and for this simple reason, 

 because the animal must have been dazzled by the 

 reflection of the bright light from the under half of the 

 latter. It covers the whole posterior portion of the inter- 

 nal eye in the cetacea and owls, many amphibia, rays, 

 and sharks, because these animals live constantly in the 

 water, or in a feebly luminous medium, or have their 

 place of residence in dark corners, or go in quest of food 

 during the night. The experiments and observations of 

 Prevost and Esser, detailed in 1826 and 1827, show that 

 the reflection of light from the tapetum is the cause of 

 the luminou?ness of the eyes, observed under certain cir- 

 cumstances in the twilight, in cats, dogs, sheep, and in 

 general in all the animals having a tapetum. But whether 

 or not a phosphoric light sometimes proceeds from the 

 retina or choroid, has not as yet been fully ascertained. 

 There are many examples of a luminousness in the dark 

 having been observed in the human eye. 



THE REASON WHY FISHES CANNOT SEE IN AIR AS WELL AS 

 IN WATER. 



When the rays of light pass from a rarer to a denser 

 medium, as from air into the aqueous humor of the eye, 

 they are refracted towards the perpendicular. Now the 



