158 



OPTICAL PRINCIPLES. 



orbit, runs along the base of the brain, and terminates at 

 last in the medulla oblongata. 



Emily. May not your description be illustrated by 

 a plate of the eye ? This would now give me a very 

 definite notion of the form and situation of the several 

 parts. O, here, you have one already. 



Dr. B. This may convey 

 a clearer idea of the matter 

 than mere verbal description. 

 It represents a vertical sec- 

 tion of the eye in the middle. 

 The external line shows the 

 cut edges of the sclerotic and 

 the white one within it of the 

 choroid coat ; in front you see 

 the edge of the cornea ap- 

 pearing like a sudden bulge 

 of the sclerotic. At a little 

 distance behind the cornea is the iris ; and between it 

 and the cornea is the aqueous humor. Behind the iris 

 you observe a white body, representing the crystalline 

 lens, and the rest of the chamber is occupied by the vitre- 

 ous humor. At the posterior portion of the eye, you 

 may see the commencement of the optic nerve. 



Emily. This is quite clear. I think I understand now 

 the structure of the eye sufficiently to comprehend the 

 uses and actions of its different parts. 



Dr. B. If you have any wish to examine the eye 

 itself, it is easy enough to procure one of an ox, or sheep, 

 or fish, from the butchers, and by first freezing it, you 

 may cut it open in various directions, and observe all its 

 parts very satisfactorily. 



Emily. I should be delighted to examine one in this 

 way, and will take the first opportunity to procure one. 

 Dr. /?. Before we can examine the functions of the 

 several parts of the eye, I must call to your recollection 

 a few of the most prominent principles of optics, or the 

 laws which regulate the transmission of light. Rays oi 



