NATURAL THEOLOGY. 



41 



part of the ball, and is kept in a round form by the 

 shape of the eye. It serves the same purpose as the 

 magnifying glasses which are used in telescopes ; it 

 being well known that a round drop of water will 

 magnify like a piece of glass of the same shape. Next, 

 and immediately behind this water glass, if we may 

 so term it, we come to a curtain, called the iris, 

 stretching entirely across from side to side, with a 

 small round hole in the middle. This hole is named 

 the pupil. We see the same in a spying-glass ; it 

 being found to increase the clearness of the sight, 

 when the light passes through a little aperture. In 

 telescopes the contrivance is what the instrument ma- 

 kers call the field. This is a plate of brass inside the 

 tube, perforated in the middle with a small round 

 hole directly in a line with the centre of the glass be- 

 fore which the field is placed. This contrivance is 

 found to improve the instrument by not permitting 



Fig. 4^5. 



A telescope glass, I, with its curtain before it, a a, in which there 

 is an aperture at the centre : being a precise imitation of the 

 eye, where the same parts are indicated in the Figure by the 

 same letters; viz. I, the principal glass of the eye, called the 

 chrystaline ; a a, the iris, with the pupil in the middle. 



d2 



