16 METHODS OF MICROSCOPICAL RESEARCH. 



individuals, and is the part referred to when we 

 speak of the colour of a person's eye. 



Behind the pupil is the crystalline lens, I, having 

 a much greater convexity at its posterior surface 

 than at the anterior. 



The large posterior chamber is lined by the 

 choroid coat, and this choroid has in front of it a 

 delicate membrane called the retina. 



The choroid coat consists of a highly vascular 

 membrane containing pigment cells, filled with an 

 intense black mucus, called the pigmentum nigrum. 



The cavity behind the cornea is filled with a 

 liquid called the aqueous humour, having a refractive 

 index approaching that of 1*3366, while the larger 

 cavity is filled with a transparent jelly, called the 

 vitreous humour, possessing a refractive index of 

 1*3379, enclosed in a very thin, transparent sac, 

 called the hyaloid membrane. 



I have now described the principal apparatus of 

 the eye, and may take some of the parts in detail. 



The crystalline lens is built up of layers, increas- 

 ing in density inwards, the effect of which is to 

 diminish spherical aberration. This lens is enclosed 

 in a transparent capsule, held in position by an elastic 

 membrane. It can be changed in shape by means 

 of a delicate muscular arrangement to adapt its 

 focus for near or distant objects. 



As glass lenses of varying curves have different 

 focal lengths, so by altering the curves of the 

 crystalline lens we are able to see objects distinctly 

 which are situated in several focal planes. 



The reader may have noticed that there is a near 

 point at which objects can be seen most distinctly ; 

 this point varies in individuals, but averages from 



