CHAP, in.] SIGHT. 1157 



but the second, that from the anterior surface of the lens, is 

 seen to become distinctly smaller, shewing that the surface has 

 become more convex. When, on the contrary, vision is directed 

 from near to far objects, the image from the anterior surface of 



abc abc abc 



FIG. 139. DIAGRAM OF IMAGES REFLECTED FROM THE EYE. 



In A are seen the three images of a candle reflected from a, the anterior surface of 

 the cornea, b, the anterior surface of the lens, and c the posterior surface of 

 the lens, a is bright and erect, b also erect, is larger but less bright, c 

 inverted is small and dim. 



B shews the images, two squares, as seen in the phakoscope when the eye is 

 directed to a far object. G the same when the eye is accommodated for a 

 near object. The pair b are in C, smaller and closer together than in B, 

 shewing an increase of curvature. 



the lens grows larger, indicating that the convexity of the surface 

 has diminished, while no change takes place in the image from 

 the cornea, and none, or hardly any, in that from the posterior 

 surface of the lens. And accurate measurements of the size of 

 the image from the anterior surface of the lens have shewn that 

 the changes in curvature which do take place are considerable ; 

 the radius of curvature of the lens accommodated for near objects 

 is 6 mm., for far objects 10 mm. ; and this difference is sufficient 

 to account for the power of accommodation which the eye 

 possesses. 



The observation of these reflected images is facilitated by the simple 

 instrument introduced by Helmholtz and called a Phakoscope. It 

 consists of a small dark chamber, with apertures for the observed and 

 observing eyes ; a needle is fixed at a short distance in front of the 

 former, to serve as a near object, for which accommodation has to be 

 made ; and a lamp or candle is so disposed as to throw an image on 

 each of the three surfaces of the observed eye. Since a change in 

 the distance between two images is more readily appreciated than is a 

 simple change of size of a single image, two prisms are employed so 

 as to throw a double image in the form of bright squares on each of 

 the three surfaces, Fig. 139 B, C. When the anterior surface of the 

 lens becomes more convex the two images reflected from that surface 

 approach each other, C, when it becomes less convex they retire from 

 each other, B. 



These observations leave no doubt that the essential change by 

 which accommodation is effected, is an alteration of the convexity of 



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