666 DEMONSTRATING REFRACTIVE EYE DEFECTS [Cn. XV 



As shown with the glass lenses it is possible to do away with 

 astigmatism by rendering the inequality of curvature uniform, 

 hence a person with astigmatism can be given normal vision by the 

 use of the proper spectacles. 



928. Correct position of spectacles. It is extremely import- 

 ant that glasses to correct astigmatism should be correctly adjusted 

 to the eyes. The necessity of this can be strikingly shown by 

 making the axes of the two cylinders in the last experiment some- 

 what oblique . When the axes are 



oblique the confusion is greater ASTIGMATISM 

 than when the correcting lens is 

 removed entirely. Not only must Thomas Young, 1800 



the correct spectacle be used, but """TJ^awSSwSl" f *"* 

 it must be correctly adjusted. 



Pp. 39-40, astigmatism of the crystalline 



Furthermore , it should be known lens - 



that the axis Of astigmatism in P. 57. astigmatism of the cornea. 



Correction by obliquity of the pctcU>. 



the eye sometimes changes so 



that the spectacles which gave DA o 



perfect vision at one time would George B. Airy, 1825 



not do SO at a later time. In Cambridge Phi.os. Trans. Vol. II (1827) 



t 1 j 1 Correction by obliquity of the p-etacli, but mot 



such a case new spectacles with ,. ,. , cyllBdriclll ,..... 



axes arranged to meet the changed FIG ^ THE DISCOVERERS OF As _ 



conditions in the eyes are neces- TIGMATISM AND THE MEANS OF 



CORRECTING IT. 

 bcii y . 



929. The two foci of astigmatic lenses, and the correction of 

 astigmatism with cylinders having the same form (both convex 

 or both concave). Use the same outfit as for 927. 



Focus sharply the image of the radial lines. Add the 0.5 diopter 

 convex cylinder and make its axis vertical. The horizontal lines 

 will be sharp. The vertical and intermediate lines will be blurred. 

 Now move the object up towards the astigmatic combination and 

 soon the vertical lines will become sharp and the horizontal and 

 intermediate lines dim. In this position the focus is for the original 

 projection lens (3 or 4 diopters) with the added curvature (0.5 

 diopter) of the cylinder. That is the greatest curvature is now 

 acting to focus the image of the vertical lines. With the horizontal 

 lines in focus, it was the least curvature which was acting. 



