SENSE OKGANS AND SENSATIONS 245 



in front of the cornea; and it is the first essential of clear 

 vision, just as it is the first essential of photography, that 

 this image be sharp, or at least distinct. A simple experi- 

 ment will show that clear vision of near and of distant ob- 

 jects cannot be had by the eye at the same time. Hold up 

 a pencil or a pen about ten inches from the eye and look 

 first at it and then at some object far away. Both can be 

 seen, but only one at a time clearly, 

 and often an effort is required to shift 

 from the far to the near object. 



The change which occurs in the 

 eye in the act of accommodation is 

 illustrated in the following experi- 

 ment: A wooden or pasteboard box 

 (approximately 8 by 5 by 4 inches) 

 is fitted with a piece of ground glass 

 on one side and provided with a con- 

 vex lens on the opposite side. This 

 is a rude camera, and some object is 

 now placed at such a distance that 

 the lens forms an image of it on the FIG. 94. Diagram of the 

 ground glass, which is now in focus mechanism of accommoda- 

 for the object. If, later, the object , 



J The ciliary muscle is repre- 



be moved nearer to the lens, the sented as three fibers passing 



focus is changed; the image on the obliquely from the sclerotic 



to the choroid 



glass becomes blurred, and m order 



to make it distinct it will be found necessary to use a 



more convex lens. 



Essentially the same change occurs in the eye in accom- 

 modating for near objects : the lens must be made more con- 

 vex; and this, it will be remembered, involves work on the 

 part of the muscle of accommodation (see p. 244). We can 

 thus understand why, in general, it is too much of "near 

 work," and especially near work necessitating very distinct 

 vision, that tires the eye. The ideal condition of the eye, 



