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feet ; however; tlie rays of light are already so little divergent; 
that they are accounted parallel; whilst at a less distance than 
eighteen feet the rays are called divergent. The course of 
parallel rays is as follows (fig. 1 7) : reflected from a, the rays 
pass to and through the cornea, where they are bent or reflected 
towards the axis of the eye (b, c) ; they then pass through the 
aqueous humour to the crystalline lens, where they are still 
more refracted, so as to cross the axis of the eye, and, traversing 
the vitreous body, terminate on the retina, where an inverted 
image of the object is formed. 
If any divergent rays from a near object were to pass through 
the eye, w h ilst the eye was in the same state as that in which 
the parallel or distant rays come to a focus, the effect would be 
that the picture of the object would form behind the retina, 
and thus perfect and well-defined sight would be impossible for 
near objects. To meet this there is a little mechanical arrange- 
ment in the interior of the eye. The ciliary muscle (Plate XI., 
fig. 8, li), attached in front to the sclerotic, and behind to the 
ciliary processes, when in action (that is, when the eye is ad- 
justed for near objects), pulls forward the ciliary processes, and 
therefore the plaitings of the hyaloid membrane ; and by means 
of the attachment of the hyaloid to the circumference of the 
lens, this body is also brought forward; the canal of Petit 
prevents the muscle acting on the vitreous body, by leaving a 
space between. By this means a greater distance is produced 
between the lens and the retina, and thus the image of the 
object is brought to a focus directly upon the retina. 
There is nothing that man can construct to equal, in beauty 
of mechanism, the iris. It is a curtain, circular in shape, and 
with an aperture in the middle, which can either contract or 
dilate without showing the least puckering or plaiting of the 
surface of the his. The contraction of the pupil takes place 
in a strong light, and is intended to exclude an excess that 
would act injuriously on the retina ; the dilatation takes place 
in dull light or darkness, for the purpose of allowing every 
possible ray to pass. 
During adjustment of the eye for a near object, it is turned 
inwards, and the same nerve by which this movement is effected 
causes contraction of the pupil. This contraction probably 
merely excludes excess of light, and does not aid adjustment 
of the eye ; as adjustment has been observed to be perfect even 
when the iris was entirely absent.* 
* For much interesting and valuable information respecting the eye, 
see the second volume of Todd & Bowman’s “ Physiological Anatomy ; ” 
Griffiths & Henfrey’s “ Micrographical Dictionary ” (Van Voorst), article 
“ Eye ”), and the books of reference there named. 
