*P EYE. 



the passage of any light but what enters about mid- 

 way of the glasses. To this structure there is a won- 

 derful resemblance in the natural telescope ; the iris 

 being the field ; and the pupil answering to the little 

 aperture in the field, it being exactly before the cen- 

 tre of the principal glass of the eye. (See Fig. 4 

 and 5.) 



But the hole in the eye, which is called the pupil, 

 is made to answer another purpose. It enlarges or 

 contracts, to suit the quantity of light, s6 that in a 

 bright sun too much light may not be admitted into 

 the eye ; which would be painful and injurious. If 

 we examine the eye of a person by bringing up a can- 

 dle, we shall see this opening diminish, as the light in- 

 creases ; or, if we darken the eye with our hands, and 

 suddenly remove them, we shall notice the same effect. 

 The curtain, which we term the iris, has, in different 

 persons, different colors around the pupil in front, ma- 

 king that painted circle we observe in the middle of 

 the eye ; in some people, black, in others, blue, &c. 

 It has been remarked, that it is generally either light 

 or dark, according to the color of the hair. Behind 

 the pupil is situated the next glass of the eye, called 

 the chrystaline humor.* This is still more rounded ; 

 and is of a gristly substance, but perfectly clear and 

 transparent ; and is the most powerful glass in the 



* A small portion of the aqueous humor is situated between th 

 pupil and the chrystaline ; but this is not of essential impor- 

 tance in the present description. One use of this intermediate 

 water is supposed to be, to moisten the iris, and enable this 

 important curtain to perform its office with more ease. 



