THE EEFRACTION OF THE EYE 



539 



ATC 



Fir,. 270. Showing the shape of foci at different positions in cases of astigmatism. 



z 



The diagnosis and measurement of astigmatism presents no difficulties. Its exist- 

 ence may be readily proved by causing the patient to look at a figure consisting of a 

 series of lines radiating from a common centre. It is then found that while some of 

 the lines are sharp those at right angles are indistinct. This test also shows the axes 

 of the principal meridians. By retinoscopy (see page 535) the axes and the amounts 

 of the abnormality in those axes may be readily determined. The treatment consists 

 in giving spectacles which have been ground on one side to a cylindrical surface. The 

 is of this cylinder is adjusted to correspond with one of the principal meridians of 

 eye of the patient. The curve given to the cylinder is that which will cause the 

 focus of the meridian with which it corresponds to be equal to the focus of the other 

 meridian. The other side of the spectacle lens is ground to that spherical surface 

 which will make the eye emmetropic after it has been corrected by the cylinder. 



ANISOMETROPIA. The last abnormality of refraction, which we have to consider, 

 is called anisometropia. It simply means difference between the refraction of the 

 two eyes. The effect on vision is very slight, since it is found that as a rule one eye 

 does all the work and the image of the other, which is necessarily indistinct, is prevented 

 from reaching consciousness. The result in course of time is that the unused eye loses 

 to a considerable extent its power of seeing and as a result strabismus develops. Treat- 

 ment consists in giving glasses which correct each eye separately, and then instituting 

 exercises for the poorer eye, in order to improve its vision. The results of this treatment 

 are good. 



