388 Pigtwritttm, Iris,&c. [Book IX, 



fubftance of a dark colour, called the pigmentum ni- 

 grum. The epithet black, however, is not defcriptive of 

 this fttbftsnce in every race of animals. On the con- 

 trary, in the ferret the pigmentum is white, and this cir- 

 cumftance enables that animal to fee in the dark, a 

 faculty well adapted to its habits and mode of life. 

 In man, diftinct vifion in a full light is a more ufeful 

 quality than the power of diftinguifhing objects where 

 the light of day is excluded. The feafon, therefore, 

 of the black colour of the pigmentum is, probably, 

 that thofe rays which pafs the retina, which is a 

 fibrous' fubftarice, may be abforbed, whereas, when 

 it is of a light colour, many of them are reflected and 

 ftrike the retina, thus increafing the power of vifion 

 where there is a deficiency of light, but producing too 

 great an effulgence and glare in ordinary cafes. This 

 reflection is very obvious in the degree of illumination 

 which proceeds from the eyes of a cat in a dark 

 place *. 



The poflerior part of the iris is of the colour of a 

 grape, and was therefore by the ancknts called the 

 uvea. The eye, being, therefore, every where pro- 

 vided within, except at the entrance of the optic nerve, 

 with a lining of a dark colour, becomes a camera ob- 

 fcura, and the light which is admitted through the 

 pupil, and pafles to the bottom of the eye, is not dif- 

 turbed with light reflected from other furfaces. 



The ball of the eye is filled with three fubftances, 

 yrmch differ from each other in confidence, but are 

 all called humours of the eye ; they are the vitreous,, 

 the cryftalline> and the aqueous. See plate xv. fig. i> 

 and 2, : 



* Hunter on the pigmentum of the eye. See his Aniraal 



' The 



