218 



THE EYE 



(4) Spherical Aberration. Rays of light passing through the 

 peripheral part of a lens are retracted more than those passing 

 through the central part. This tends to produce distortion of 

 the image at the periphery. 



We will point out the application of these facts to the eye 

 in due course. 



Anatomy. 



To understand the mechanism of the eye even from the purely 

 physical standpoint, i.e. as .an optical instrument, it is necessary 

 to have a clear conception of its'structure. 



IRJS 



CORNER 



CONJUNCTIVA 



FILTRATION ANGLE 



. CILIARY 



MUSCLE 



'.SCLEROTIC 



OPTIC NEKVE1 



FIG. 49. Diagrammatic section of eye. 



The eyeball is a hollow sphere of dense fibrous tissue the 

 sclerotic. In front there is a window, the cornea, formed also of 

 fibrous tissue, but so modified as to be transparent. Its radius 

 of curvature is much smaller than that of the sclerotic. Inside 

 the eyeball is a second layer, the choroid, constituting the vascular 

 portion of the eye, and loaded with pigment. This last performs 

 the function of the black lining of optical instruments, preventing 

 the reflection of rays in the interior. At the corneo-sclerotic 

 junction the choroid ceases to be in contact with the wall of the 

 eyeball, and hangs free as a curtain the iris. In this curtain 

 there is a central aperture, the pupil, which in the human subject 

 is circular. In the iris are muscular fibres, a well-marked circular 

 set serving to diminish, and a feebly marked radial set serving 



