. On the anomalies of accommodation and refraction of the eye, witha preliminary essay on physiological dioptrics. ed as a single refracting surface, with adifference of radius of curvature in its several meridians, and it willmore fully appear that wre are justified in so considering it. Moreover, through the form of the diffusion-images, in refractionby such a surface, what I have stated respecting the difference in thedistance of distinct vision of stripes of different direction is fullyexplained. Horizontal and vertical stripes, namely, are acutely seen,when the diffusion-images of all the
. On the anomalies of accommodation and refraction of the eye, witha preliminary essay on physiological dioptrics. ed as a single refracting surface, with adifference of radius of curvature in its several meridians, and it willmore fully appear that wre are justified in so considering it. Moreover, through the form of the diffusion-images, in refractionby such a surface, what I have stated respecting the difference in thedistance of distinct vision of stripes of different direction is fullyexplained. Horizontal and vertical stripes, namely, are acutely seen,when the diffusion-images of all the points of the stripe form respec-tively horizontal and vertical lines, which cover one another in thestripe; and this will be the case when the beginning and the endof the focal interval correspond respectively to the percipientsurface of the retina. To make the description easier, we have thus far assumed that themaximum of curvature coincides with the vertical, the minimum withthe horizontal meridian. And the rule in fact is, that they nearlydo so. But to this rule there are numerous exceptions. Not unfre-. 454 ASTIGMATISM. quently the deviation from the ordinary direction is very considerable ;and it even occurs that the maximum of curvature coincides nearlywith the horizontal, the minimum with the vertical meridian. 80Thomas Young, the discoverer of astigmatism, found it in his owneye, and I too have met with some cases of this nature. In general there is no difficulty in determining the direction of theprincipal meridians (those of the maximum and minimum of curva-ture) . The mode of doing so is included inthe experiments above described, in proof ofthe existence of astigmatism. Were weso perfectly conscious of our accommoda-tion that we could accurately state, whatlines in the annexed figure are seen quitesharply at the maximum, and what at theminimum of augmented tension, the direc-tions of the maximum and minimum of cur-vature would at the same time be known. That cons
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