766 THE EYE AND VISION. [CH. LV. 



into its original white light, corrects any chromatic aberration 

 which may have resulted from the first. It is probable that the 

 unequal refractive power of the transparent media in front of the 

 retina may be the means by which the eye is enabled to guard 

 against the effect of chromatic aberration. The human eye is 

 achromatic, however, only so long as the image is received at its 

 focal distance upon the retina, or so long as the eye adapts itself 

 to the different distances of sight. If either of these conditions 

 be interfered with, a more or less distinct appearance of colours is 

 produced. 



From the insufficient adjustment of the image of a small white 

 object, it appears surrounded by a sort of halo or fringe. This 

 phenomenon is termed Irradiation. It is for this reason that a 

 white square on a black ground appears larger than a black sq uare 

 of the same size on a white ground. 



Defective Accommodation Presbyopia. This condition is due 

 to the gradual loss of the power of accommodation which is an 

 early sign of advancing years. In consequence the person 

 is obliged in reading to hold the book further and further away in 

 order to focus the letters, till at last the letters are held too far for 

 distinct vision. The defect is remedied by weak convex glasses. 

 It is due chiefly to the gradual increase in density of the lens, 

 which is unable to swell out and become convex when near 

 objects are looked at, and also to a weakening of the ciliary 

 muscle, and a general loss of elasticity in the parts concerned in 

 the mechanism. 



FUNCTIONS OF THE IRIS. 



The iris has three uses : 



1. To act as a diaphragm in order to lessen spherical aberra- 

 tion in the manner just described. 



2. To regulate the amount of light entering the eye. In a 

 bright light the pupil contracts ; in a dim light it enlarges. 

 This may be perfectly well seen in one's own iris by looking at 

 it in a mirror while one alternately turns a gas-light up and 

 down. 



3. By its contraction during accommodation it supports the 

 action of the ciliary muscle. 



The muscular fibres (unstriped in mammals, striped in birds) 

 of the iris are arranged circularly around the margin of the 

 pupil, and radiatingly from its margin. The radiating fibres are 

 best seen in the eyes of birds and otters ; some look upon them 

 as elastic in nature, but there is little doubt that they are con- 

 tractile. Those who believe they are not contractile explain 



