ACCOMMODATION. 105 



of diffusion are removed that constitutes accommodation. 

 This fact has been demonstrated by Helmholtz by means of 

 the ophthalmoscope. " If the eye be adjusted to the obser- 

 vation of an object placed at a certain distance, it is found 

 that the image of a flame, placed at the same distance, is 

 produced with perfect distinctness upon the retina, and, 

 at the same time, upon the illuminated plane of the image, 

 the vessels and the other anatomical details of the retina 

 are seen with equal distinctness. But, when the flame is 

 brought considerably nearer, its image becomes confused, 

 while the details of the structure of the retina remain per- 

 fectly distinct." * 



It is evident that there is a certain condition of the eyes 

 adapted to vision at an infinite distance, and that, for the 

 clear perception of near objects, the transparent media must 

 be so altered in their arrangement, or in the curvatures of 

 their surfaces, that the refraction will be greater ; for, without 

 this, the rays would be brought to a focus beyond the retina. 

 It is the mechanism of this adjustment to distances that has 

 engaged the attention of physiologists for so many years. 



To produce the necessary changes in the refractive power 

 of the eye, physiologists have supposed a variety of conditions, 

 none of their theories, however, presenting any positive basis 

 in fact. It was thus with the theory that contraction of the 

 pupil was sufficient for accommodation ; that the curvature of 

 the cornea was increased for near objects ; that the lens was 

 displaced, as in the adjustment of ordinary optical instru- 

 ments ; and the idea that the antero-posterior diameter of the 

 eye was modified. The theory that has since been proven 

 to be correct ; namely, that of changes in the curvatures of 

 the crystalline lens, was advanced by Thomas Young ; 3 but 

 the mechanism of these changes, supposed by Young to be 

 due to the action of the lens itself, the constituent fibres of 



1 HELMHOLTZ, Optique physiologique, Paris, 1867, p. 123. 



2 YOUNG, Observations on Vision, JRead before the Royal Society, London, May 

 80, 1793 ; and, On the Mechanism of the Eye, London, 1801. 



