
THE ELEMENTS OF MICROSCOPY.—THE HUMAN EYE. 295 
except in front, and this is lined with a delicate membrane, called 
the retina, shown at K. 
The choroid coat consists of a highly vascular membrane con- 
taining pigment cells, filled with an intense black mucus, called 
the pigmentum nigrum. 
The cavity of 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, increasing 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 
ea muscular arrangement to adapt its focus for near or distant 
° 
You will all know that glass lenses of varying curves have diffe- 
rent focal lengths, and it is by a process of altering the curves of 
the crystalline lens that we are able to see objects distinctly which 
are situated in several focal planes. 
It has, perhaps, been noticed by all of you that there is a near 
point at which objects can be seen most dis- 
tinctly ; this point varies in individuals, but 
averages from 8 to 10 inches. As we move far- 
ther away from the object, although diminished 
in size, it may be seen more easily and with 
less effort. 
It would seem, then, that all objects are 
rendered apparently larger as they continue 
to approach the eye, but a limit is soon found 
to this, as at a distance of six inches distinct 
and easy vision is not possible (except in very 
abnormal cases). 
The reason of this is well-known—the an- 
eed pe cam of a convex lens when act 
en € posterior conju ocus, 
so chet eet te object is brought coorbien the 
eye the i of it is projected behind the 
retina, and the crystalline lens cannot accom- 
modate itself to such extremes. But we know 
that objects can be seen distinctly at great dis- 
tances apart, and it may be useful to demon- 
Strate how this is brought about. 
The figure (23) represents a cross section of Fig. 23. 



