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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 important 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 somewhat 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 •p»ct«cU>.
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«-etacl«i», but mo«t
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.