40 EYE. 



but they will always leave the eye, it is likely, the 

 most finished instrument of the kind, that ever was or 

 ever can be made. 



J5. One must be quite learned, then, about teles- 

 copes to understand the construction of the eye. 



T. The more learned the better ; but every one 

 must understand enough for the present purpose. If 

 you have ever pulled out the tube of a spying-glass, 

 and unscrewed the joints, you know it contains seve- 

 ral little rounded glasses ; it is these glasses that 

 make the telescope. The eye is just such another 

 instrument ; so that when we are espying, we may 

 say there are two telescopes, the one in our hand, 

 and the other in our head ; and the use of the one 

 without, is to assist the one which is within in seeing 

 further. They are both constructed on the same 

 principles. You may describe the eye. 



A. The principal parts of the eye are, as in other 

 telescopes, rounded glasses, or substances which re- 

 semble them, and which answer the same end, though 

 they are not of glass. They are all situated in the 

 ball. The front of the ball is covered with a transpa- 

 rent skin, which is termed the cornea, from a Latin 

 word signifying horn, because it has a certain resem- 

 blance to a very delicate shaving of this substance. 

 It is a little thicker in the middle than towards the 

 edges, and has in a degree the same effect as a mag- 

 nifying or telescope glass. Next to this is a fluid, 

 commonly called the aqueous humor, and which will 

 run out when the eye is pricked. It derives its name 

 from the Latin word for water. It fills the forward 



