572 MANUAL OF PHYSIOLOGY. 



gent before they strike the eorneal surface, and thus enable them 

 to be sooner brought to a focus by the dioptric media of the eye. 

 Presbyopia is the name given to a change in the perfectness of 

 accommodation frequently accompanying old age. The lens 

 probably gets less elastic and the ciliary muscle weaker, so that 

 the change in form required to see near objects is more difficult 

 or impossible to attain. Biconvex lenses help to overcome the 

 difficulty. 



FIG. 226. 



Showing the course of the rays of light from two luminous points to the 

 retina. The rays from the point a, on passing through the cornea, lens, 

 etc., are collected on the retina at 6. Those from a,' meet at &', and thus 

 the lower point becomes the upper. 



DEFECTS OF DIOPTRIC APPARATUS. 



In common with all dioptrical instruments the eye has certain 

 optical defects which tend to interfere with the exact definition 

 of the image. 



Chromatic aberration is due to the breaking up of white light 

 into colored rays owing to the different colored lights, of which 

 ordinary light is composed, possessing different degrees of refran- 

 gibility. We know this in the spectrum and in the colored rings 

 always seen in the marginal part of a biconvex lens made of one 

 kind of glass, which acts like a prism. It can be corrected by 

 making lenses of two kinds of glass, one of which counteracts 

 the dispersion caused by the other. Optical instruments may 

 thus be made achromatic. This defect is minimized by the iris, 

 which cuts off the marginal rays in which it is most apt to occur. 

 Possibly the different density of the dioptric media may have a 



