170 



SATURDAY LECTURES. 



Fig. 2. « 



You have in the first place tlie outer box or shell in the 

 sclerotic coat, (S,) which is a tough membrane forming the 

 posterior four-fifths of the ball. The anterior, one-fifth, is 

 formed by the cornea, (C,) which is only a continuation of 

 the sclerotic with the remarkable and highly important dif- 

 ference that it is transparent. This cornea being a curved 

 surface, acts upon the light in the same manner as the lens 

 in Fig. 1, and forms one of the two refracting surfaces which 

 produce the images of objects. in the external world upon 

 the expansion of the optic nerve (On) at the back of the 

 eye called the retina, (R.) The other refracting body is 

 the crystalline lens, (L.) The iris (I) represents the dia- 

 phragm, and the hole in it is the pupil. The choroid coat, 

 (Ch,) while holding the greater part of the blood-vessels 

 which nourish the eye, also contains a large quantity of 

 pigment, which serves the purpose of the black lining of 

 the photographer's camera. We have thus all the essential 

 parts of a perfect optical instrument, and by the large ma- 

 jority of persons the eye is considered the most nearly per- 

 fect of all instruments of its class from a. merely physical 



