sistirig of two globes, an upper and a lower one, 

 attached together. It appears that the superior 

 part of the eye is, like that of terrestrial animals, 

 adapted to refract rays transmitted hy air, and 

 the inferior part, like that of aquatic animals, 

 those transmitted by water ; and that the refrac- 

 ting power of the several parts of the eye is ac- 

 cordingly much less above than below. It remains 

 only to remark, that in some fishes, as the skate 

 and shark, there is, as in insects that fly by night, 

 a resplendent substance at the bottom of the eye- 

 ball, instead of the black pigment which is usu- 

 ally found there ; its use being rather to increase 

 than diminish the number of rays which fall 

 upon it. 



The eyes of reptiles in general do not differ 

 materially from those of fishes, except that they 

 appear to possess the power, of which those of 

 fishes are destitute, of adapting themselves to 

 refract rays as transmitted either by air or by 

 water. I have already hinted when speaking of 

 the singular eye of the Surinam sprat, that the 

 refracting power required is different in these 

 two cases, as any one may satisfy himself by at- 

 tempting to distinguish minute objects placed 

 in water, with his head likewise immersed in 

 this fluid. The reason that he cannot do this 

 is because, though there is a sufficient differ- 

 ence between the density of the humours of 

 his eye and that of the air, to bring the rays 

 transmitted by the latter to a focus on the re- 

 tina, there is not a sufficient difference between 



