WHEN the refracting power of the eye as a whole, instead of being identical in all meridians, changes gradually from one meridian to the next by uniform increments, and when each meridian has a uniform type of curve throughout the refracting zone, the condition is known as regular astigmatism. This is the condition that ordinarily is corrected by a cylindric or spherocylindric spectacle lens.Regular astigmatism usually is caused by a toroidal, instead of a spherical, curvature of one or more refracting surfaces of the eye. Occasionally it results from an oblique or tilted position of the lens with respect to the optic axis. Other causes are rare.When two or any larger number of cylindric or spherocylindric refracting surfaces are combined with their axes in varying positions with respect to each other, the result always will be a spherocylindric equivalent, in which the meridian of greatest curvature
EGGERS H. REFRACTION BY THE ASTIGMATIC EYE. Arch Ophthalmol. 1946;35(4):346–353. doi:10.1001/archopht.1946.00890200353002
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