530 



VISION 



We have here to distinguish between range of accommodation and line 

 of accommodation. The former,, as just stated, is the measure of the increase 

 in refracting power which can be brought about by accommodation., whereas 

 the latter gives that depth of space within which the eye can form by the help 

 of accommodation a clear picture on the retina. The range of accommodation 

 is entirely independent of the static refraction of the eye; but the line of 

 accommodation varies considerably in eyes of different refractive conditions. 



The range of accommodation decreases gradually with age, as the following 

 table compiled by Bonders will show: 



This gradual diminution in the optical power of the eye is called presby- 

 opia. It must not be confused with hypermetropia, for hypermetropia is a 



FIG. 224. After Helmholtz. 



particular kind of static refraction, while presbyopia is caused by a loss in 

 the power of accommodation. 



Presbyopia is treated with convex lenses, the strength being so chosen 

 that rays proceeding from a point lying conveniently near the eye appear to 

 come from the actual near point. Objects can then be held at any distance 

 and be clearly focused on the retina. 



B. MECHANISM OF ACCOMMODATION 



The change in the optical apparatus which takes place in accommodation 

 consists in an alteration of the form of the lens. This view was first expressed 

 by Descartes (1637), but the first conclusive proof of it was furnished more 

 than two hundred years later by Max Langenbeck, Cramer, and Helmholtz 

 (1849-1854). 



We have already seen (page 522) that the radius of curvature of a spherical 

 refracting surface can be calculated from the size of an image reflected from 



