- OPTICS. 125 
send its rays to some other point of the retina. The greater the number of 
cones the greater the clearness of the image. The transparent cornea 
coating externally the summits of these cones, is divided into a great number 
of facets, whose number, in some eyes, amounts to from twelve to twenty 
thousand, each one corresponding to the truncated cone just described. The 
size of the field of vision depends upon the angle which the axes of the outer- 
most cones make with each other. 
In simple eyes, the images are produced in the same way as the convergent 
images of convex lenses. The rays proceeding from any point and passing 
through the anterior portion of the eye, are refracted to a point in the retina. 
The following is the structure of the human eye as shown in pl. 21, jig. 50. 
The ball of the eye is inclosed in a tough, opake, white membrane, called 
the tunica sclerotica, which is anteriorly replaced by the transparent cornea. 
Immediately behind the latter is seen the colored iris, whose central perfu- 
ration, ss’, is called the pupil. Behind the pupil, and inclosed in a transparent 
membrane, is the crystalline lens, cc', most convex posteriorly. Between 
the crystalline lens and the cornea is a transparent liquid called the aqueous 
humor. The internal cavity of the eye, behind the lens, is occupied by a 
gelatinous liquid of perfect transparency, called the vitreous humor. This 
is inclosed in a capsule, subdivided by numerous partitions. The choroid 
membrane lines the inside of the sclerotic coat, and is itself invested with a 
black coating called pigmentum nigrum. Lining the choroid, with its 
pigmentum nigrum, is the retina,—a delicate expansion of the optic nerve. 
All rays impinging upon the eye fall either on the sclerotica (the white 
of the eye) and are then dispersed irregularly in every direction, or they 
penetrate the cornea. Of these, the most external meet the iris and are 
absorbed, the central ones only passing through the pupil to be refracted 
still further by the crystalline lens and vitreous humor. Rays of light, then, 
proceeding from the individual points of an object, are refracted to a point 
on the retina, producing an inverted image. In the figure, m is the image 
of the point /, m’ that of /’. All objects not too near the eye are seen with 
distinctness ; there is, however, a limit, within which the images of objects 
become confused. This, which for ordinary eyes amounts to about five 
inches, is called the limit of distinct vision. The indistinctness is pro- 
duced by the great divergence of rays proceeding from objects in very 
close proximity, and their refraction towards a point posterior to the retina. 
Although the ordinary eye can see distinctly at a distance of five inches, 
yet the ocular examination of minute objects, as the letters of a book in 
reading, is generally performed at a distance of from ten to fourteen inches. 
Persons who are obliged to hold objects much nearer than this to the eye, are 
said to be short-sighted ; and if at a greater distance, long-sighted. These 
defects of vision are remedied by the use of lenses ; concave being required 
for the first. convex for the last. 
Achromatism in the eye is effected in the same manner as in lenses, 
light traversing three different media. This achromatism, although not 
complete in all cases, is yet so nearly so as to answer all necessary purposes. 
The apparent size of an object depends upon the amount of the angle 
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