852 ACCOMMODATION. [Book hi. 



third very dim, c, by the posterior surface of the lens ; when the 

 images are those of an object, such as the flame of a candle, in 

 which a top and bottom can be recognized, the two former images 

 are seen to be erect, but the third inverted. When the eye is 

 accommodated for near objects, no change is observed in the 

 first, and none, or a very insignificant one, in the third of these 

 images ; but the second, that from the anterior surface of the 

 lens, is seen to become distinctly smaller, shewing that the sur- 

 face has become more convex. When, on the contrary, vision 

 is directed from near to far objects, the image from the anterior 



1 ^m > ~T~ J | —r i 



abc abc a b c 



Fig. 144. Diagram of Images reflected from the Ete. 



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. C 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. 



surface of 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 measure- 

 ments 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 



