Pff 1 -'SIOL OGICA L PH I 'S/CS. [Chap. XXI X . 



are pierced close to one another. The card is 

 held close to the eye, and in front of it is held 

 a needle. On moving the needle nearer to the card 

 and then farther from it, a position is found where it 

 is distinctly seen. If it be brought slightly nearer, 

 the needle appears double, and the same thing happens 

 if it be moved away a little from its first position. 

 The explanation is evident from Fig. 172, where A 



and B represent the 

 holes in the card, a the 

 point of the needle ; c 

 represents a lens, and D, 

 E, and F, a screen at 

 Pig. 172. Schemer's Experiment varying distances from 



it. With the screen 



at E, a distinct single image of the needle is perceived, 

 because the rays from A and B coincide, and are 

 focussed at o ; at the position F, the image is blurred 

 and double, because the rays from A do not coincide 

 with those from B, while at D the image is also double 

 and blurred, because the rays are intercepted after 

 they have diverged from their focus. With the 

 screen in a fixed position, the same effects are pro- 

 duced by varying the distance of a from the screen. 

 Let C and the screen represent the refractive media 

 of the eye and the retina, the explanation applies, and 

 the phenomena of diffusion images are understood. 



It is evident, then, that the eye in its condition 

 for focussing parallel rays will produce on the retina 

 images of diffusion with divergent rays, because the 

 focal point is thrown behind the retina. It is equally 

 evident that if an increase of refractive power were 

 given to the media, the focal point would be brought 

 forward and made to coincide with the retina. Every 

 different distance of the object looked at would 

 require a new adjustment. The increased refractive 

 power would be conferred by the addition of another 



