560 STUDIES IN ADVANCED PHYSIOLOGY, 



the eye depends upon the elasticity of the lens. But this 

 elasticity becomes gradually reduced and in old age fre- 

 quently disappears altogether. Such persons are unable to 

 accommodate their eyes for far and near, a defect familiarly 

 called old-sight. 



To remedy this it is obvious that several pairs of lenses 

 are required to adjust the eye to several distances. Such 

 persons will not infrequently have one system of lenses for 

 far objects, a second system for relatively near objects, the 

 objects in a room, for instance, and a third system to be 

 pressed into service in such acts of vision as the reading of 

 a printed page. 



THE MANIPULATION OF THE ETE AS AN OPTICAL INSTRUMENT. 



Every optical instrument must have some arrangement 

 by means of which it can be focussed. In the photograph 

 camera this focussing consists in moving the lens forward 

 or backward, or in changing the relative position of the 

 sensitive plate behind the lens. The microscope is focussed 

 by lifting the tube of the microscope or dropping it closer 

 to the object on the stage. In the telescope the focus is 

 produced by shifting the relative positions of the lenses. 

 Evidently there must be in the eye some arrangement by 

 means of which it, also, may be focussed. 



That such is the case is the commonest experience of 

 every-day life. We are conscious of a change in the eye 

 when we turn our view from a distant object to one close 

 by, and this change in the eye is independent of any move- 

 ment of the eyeball. We may close one eye and then look 

 at two points lying in the same direction so that there is no 

 turning of the eyeball, and yet a change from the focus of 

 one to the focus of the other is a clearly perceivable one. 

 This change in the eye is called the accommodation of the 

 eye, and the question arises, how this is brought about. 



HOW DO WE FOCUS THE EYE f 



It is at once out of the question to think this is brought 

 about by a lengthening of the eyeball, like a camera, or the 



