686 SPECIAL SENSES 



ACCOMMODATION OF THE EYE FOR VISION AT DIFFERENT DISTANCES 



Supposing the eye to be adapted to vision at an infinite distance, in 

 which the rays from an object as they strike the cornea are practically 

 parallel, it is evident that the foci of the rays, as they form a di'stinct 

 image upon the retina, are all situated in the proper plane. Under 

 these conditions, in a normal eye, the image, appreciated by the in- 

 dividual or seen by means of the ophthalmoscope, is clear and distinct. 

 If the foci are situated in front of the retina, the rays, instead of com- 

 ing to a focus on a point in the retina, will cross, and, from their diffu- 

 sion or dispersion, will produce indistinct vision. Under these conditions 

 a distinct point is not perceived, but every point in the image is sur- 

 rounded with an indistinct circle. These are called " circles of diffu- 

 sion." If, now, the eye, adjusted for vision at an infinite distance, is 

 brought to bear on a near object, the rays from which are divergent as 

 they strike the cornea, the image will no longer be distinct but will be 

 obscured by circles of diffusion. It is the adjustment by which these 

 circles of diffusion are removed that constitutes accommodation. This 

 fact has been demonstrated by Helmholtz by means of the ophthalmo- 

 scope. " If the eye is adjusted to the observation 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 perfectly distinct." 



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

 vision at an infinite distance ; and that for the distinct perception of 

 near objects the transparent media must be so altered in their arrange- 

 ment 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. 



The changes in the eye by which accommodation is effected are now 

 known to consist mainly in an increased convexity of the lens for near 

 objects ; and the only points in dispute are a few unimportant details in 

 the mechanism of this action. The simple facts to be borne in mind in 

 studying this question are the following : - 



When the eye is accommodated to vision at an infinite distance the 

 parts are passive. 



In the adjustment of the eye for near objects the convexities of the 

 lens are increased by muscular action. 



