THE EYE AND THE PHYSIOLOGY OF VISION. 565 



pupil is accomplished by the radial fibers of the iris, its 

 contraction by the circular ones. The pupil changes its 

 caliber under the following circumstances: 



First. Very strong light thrown into the eye causes a 

 contraction of both pupils, even when the strong light is 

 admitted to one eye only. 



Second. Under a constant light the pupil contracts 

 when the eye is accommodated for objects near by. 



Third. A contraction of the pupil seems to accompany 

 the movements of the eyes inwards. This is probably but 

 a special case of number two, as ordinarily the eyes are 

 turned inward when they are accommodated for near ob- 

 jects. 



Fourth. The pupil is contracted in sleep. 



Fifth. The pupil is much dilated in cases of asphyxia- 

 tion. A dilation also usually follows a strong stimulation 

 of any large region of the skin of the body. 



Sixth. Various drugs have a decided effect upon the 

 pupil. Thus, atropin and belladonna dilate the pupil, 

 while nicotine and morphine cause it to contract. 



THE DIOPTRICS OF THE ETE. 



In a general way the passage of rays of light through 

 the media of the eye has been pointed out, and while it 

 would be out of place here to go into the detailed mathe- 

 matical considerations of the various foci and indices of re- 

 fraction a few elementary considerations of this nature seem 

 appropriate. Although the eye consists of several refract- 

 ing media, these three media may mathematically be re- 

 solved into one, and the cardinal points for such a com- 

 posite medium be determined. In Figure 178 the line // 

 actually indicates the curvature of a lens of the density of 

 the vitreous humor which would have the same refracting 

 power as the entire eye. To determine at what point on 

 the retina the refracting media of the eye will throw an 

 image it is necessary that several determining points of 

 these media be established. These have, with considerable 



