600 



PHYSIOLOGY 



of vision may be determined in any eye by finding out the smallest 

 distance from the eye at which small print can be easily distinguished. 

 The distance is measured by means of a graduated rod between the 

 eye and the printed object. This ' accommodation, ' by which the 

 eye is able to focus divergent rays on to the retina, implies either a 



FIG. 265. Diagram of phakoscope. 



change in the distance of the refracting surfaces from the retina, or 

 an increase in the total refractive powers of the eye. 



In man and the higher animals it is by the latter means alone 

 that 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 



a b c a b c 



FIG. 266. Diagram of reflected images from cornea and lens surfaces seen 



in phakoscope. 



a, from anterior surface of cornea ; b, from anterior surface of lens ; 

 c, from posterior surface of lens. 1, during accommodation for distance ; 

 2, during accommodation for near objects. 



change in the combination cannot be located in the cornea. It was 

 shown by Helmholtz that the essential process in accommodation 

 is an alteration in the curvature of the lens, the anterior surface 

 becoming more convex when the eye is accommodated for near objects. 



This may be shown by means of the phakoscope (Fig. 265). 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 



