574 MANUAL OF PHYSIOLOGY. 



vergent before they strike the corneal 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. 



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 

 correcting effect on the chromatism of the eye. Further correc- 

 tion takes place in the nerve centres which receive the sensation, 

 for just as we mentally reinvert the image, we fail to see the 

 color. At any rate, the chromatic aberration is so slight that it 

 needs certain artifices to make it observable. 



Spherical aberration depends upon the fact that luminous rays, 

 on passing through a convex lens, strike the various parts of its 

 surface at different angles, and hence are differently refracted. 

 The rays striking the margin of the lens are more bent than those 

 passing through the centre, and hence the former come sooner to 

 a focus. Thus a luminous point gives rise to a diffused figure, 

 which is circular in perfectly centred dioptric systems, or stellate 

 in our eyes where the centring of the lenses is not absolutely 



