CH. LV.] 



THE PHAKOSCOPE. 



761 



Range of Distinct Vision. Near-point. In every eye there is 

 a limit to the power of accommodation. If a book be brought 



Fig. 582. Fhakoscope of Helmholtz. At B B' are two prisms, by which the light of a 

 candle is concentrated on the eye of the person experimented with, which is looking 

 through a hole in the third angle of the box opposite to the window C. A is the 

 aperture for the eye of the observer. The observer notices three double images, repre- 

 sented by arrows, in fig. 581, reflected from the eye under examination when the eye is 

 fixed upon a distant object, the position of the images having been noticed, the eye 

 is then made to focus a near object, such as a reed pushed up at C ; the images from 

 thf. anterior surface of the lens will be observed to move towards each other, in 

 consequence of this surface of the lens becoming more convex. 



nearer and nearer to the eye, the type at last becomes indistinct, 

 and cannot be brought into focus by any effort of accommodation, 



Fig. 583. Diagram representing by dotted lines the alteration in the shape of the lens on 

 accommodation for near object*. (. Landolt.) 



