64 How some animals see in the dark. 



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 ocufar 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 this 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 day light, 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 lat- 

 ter. It covers the whole posterior portion of the internal eye 

 in the cetacea and owls, many amphibia, rays, and sharks, be- 

 cause 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 experi- 

 ments and observations of Prevost and Esser, detailed in the 

 numbers of this Journal for October 1826 and January 1827, 

 shew that the reflection of light from the tapetum is the cause 

 of the luminousness of the eyes observed under certain circum- 

 stances in the twilight in cats, dogs, sheep, and in general in all 

 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 lu- 

 minousness in the dark having been observed in the human eye, 

 (a case of this kind we noticed, and recorded in a former num- 

 ber of the Journal,) yet it wants the tapetum. Probably, as 

 Treviranus remarks, such cases may be of a pathological nature. 



3. On seeing at a distance. 



When we speak of the distance to which vision extends, we 

 can understand, as Treviranus remarks, either the sphere of dis- 



