540 



PHYSIOLOGY 



FIG. 267. Diagram of phakoscope. 



accommodation is effected. The fact that accommodation, as was shown 

 by Young, may be carried out under water, i.e. under conditions in which the 

 curvature of the cornea does not cause any appreciable deviation of the rays 

 passing through it, shows that the change in the combination cannot be 

 located in the cornea. It was shown by Helmholtz that the essential process 



in accommodation is an altera- 

 tion in the curvature of the 

 lens, the anterior surface be- 

 coming more convex when the 

 eye is accommodated for near 

 objects. 



This may be shown by means 

 of the phakoscope (Fig. 267). 

 This is simply a box, blackened 

 inside, with holes at a, b, c, 

 and d. At a is the observer's 

 eye ; at b the observed eye. Across the middle of d a wire is stretched. 



A candle is placed at c. The observer at a then sees three reflections 

 of the candle from the eye at b : a bright erect image from the anterior 

 surface of the cornea ; a larger but dimmer erect image from the anterior 

 surface of the lens ; and a small very dim inverted image from the 

 posterior surface of the lens. These images must be observed first when 

 the eye at b is accommodated for a 

 distant object, and then when it is 

 accommodated for the wire stretched 

 across the opening d. It will be 

 noticed that the change of accom- 

 modation from far to near objects is 

 accompanied by a change in the 

 second image (that from the anterior 

 surface of the lens), which becomes 

 smaller. The change in this image 

 is more easily seen if the candle be 6> from anterior surface of Ien8; ^ from 



made to throw two images On the posterior surface of lens. 1, during accom- 

 T , 111 modation for distance ; 2, during accom- 



eye by interposing a double prism modation for near objects. 



at c. Then, as the lens becomes more 



convex to accommodate for near objects, the two images of the candle 



reflected from its anterior surface approach one another (Fig. 268). 



By measuring the size of the image of the candle-flame produced by the 

 anterior surface of the lens, and knowing the size of the flame itself and the 

 distance from the observed eye, it is possible to calculate the curvature of the 

 lens in the living body. 



The radius of curvature of a reflecting surface is given approximately by the follow- 

 ing formula : 



R 2 a ' b 

 R=2 



a b T! a b c 



FIG. 268. Diagram of reflected images 

 from cornea and lens surfaces seen in 

 phakoscope. 



a, from anterior surface of cornea 



