52 EYE. 



one substance to another, should find a sudden differ- 

 ence. Such is the structure of the eye ; discovering 

 such perfect acquaintance with some of the most 

 curious properties of light ; and uniting philosophi- 

 cal contrivances, various, as they are admirable and 

 inimitable. 



T. Speaking of the reflection of the light, do you 

 understand why the inside of a spying-glass is always 

 painted black around the glasses ? 



A. To prevent the glistening of that portion of 

 the light which strikes against the sides of the tube. 

 There is a similar provision in our eyes. The inside 

 lining on the back part of the eye, called the choroid 

 coat, is blackened over with a substance termed the 

 pigmentum nigrum, black pigment, or paint. In 

 many animals, such as cats, owls, &c, this paint is of 

 a bright color which reflects the light. The precise 

 object of the difference seems yet to be a subject of 

 inquiry. Some have conjectured that it enables the 

 animal to see better in the dark. It has, no doubt, 

 some design. It is ascertained in our own eyes, that 

 if from any cause the black pigment happens to be 

 wanting, the sight is injuriously affected. The sin- 

 gular persons called Albinos, from a Latin word sig- 

 nifying white, have no such pigment in their eyes ; 

 and it is found their vision is extremely imperfect in 

 a bright light ; so that they can scarcely see how to 

 direct themselves. 



