PHAKOSCOPE OF HELMHOLTZ. 



269 



(c.) Physical Experiment. Place in a convenient position 

 on a table a large convex lens, supported on a stand. Stand- 

 ing in front of it, hold a watch-glass in the left hand in front 

 of the lens and a few inches from it. Move a lighted candle 

 at the side of this arrangement, and observe the three images 

 described above. Substitute a convex lens of shorter focus, 

 and observe how the images reflected from the lens become 

 smaller. 



9. The Phakoscope of Helmholtz is used to demonstrate more 

 clearly the change in the curvature of the crystalline lens during 

 accommodation (Fig. 123). 



(a.) Place the phakoscope in a convenient position, darken 

 the room. Two persons are required. The observed eye 

 looks through a hole in 

 the box opposite to c, while 

 the observer looks through 

 the hole (a) at the side. 

 Light a lamp, place it some 

 distance from the two 

 prisms (b, b') in such a posi- 

 tion that its light is thrown 

 clearly upon the observed 

 eye, and the observer sees 

 two small bright square 

 images of light, when the 

 observed eye looks straight 

 ahead at a distant object. 

 These are the corneal 

 images. He should also 

 see in the observed eye, 

 two larger less distinct 

 images, from the anterior 



surface of the lens, and 

 two smaller and much 

 dimmer images, from the 

 posterior surface of the 

 lens. The last are seen 

 with difficulty. 



Hole for 



Fig. 123.- 



observer's eye 



carries a needle for the observed 



eye to fix as its near point. 



-Phakoscope. 



; b, b', prisms ; c, 



(b.) Ask the observer to accommodate for a near object 

 viz., the pin above c, keeping the eye unmoved. Observe 

 that the middle image becomes smaller and goes nearer 



