. The clinical use of prisms; and the decentering of lenses . ng from, orgoing towards, a single point, is, after traversinga prism, no longer homocentric. The stronger theprism, and the greater the departure of the beamof light from the direction of minimum deviation,and the more converging or diverging its com-ponent rays, the greater is the prismatic astig-matism. With weak prisms, as used in practice,it gives rise to little more inconvenience than thespherical aberration of lenses. But it accounts for some phenomena, which areapt to puzzle those who experiment with strongprisms. The astigm
. The clinical use of prisms; and the decentering of lenses . ng from, orgoing towards, a single point, is, after traversinga prism, no longer homocentric. The stronger theprism, and the greater the departure of the beamof light from the direction of minimum deviation,and the more converging or diverging its com-ponent rays, the greater is the prismatic astig-matism. With weak prisms, as used in practice,it gives rise to little more inconvenience than thespherical aberration of lenses. But it accounts for some phenomena, which areapt to puzzle those who experiment with strongprisms. The astigmatism of a prism is slightlyakin to that produced by a weak cylindrical lens,in that a cone of light, after having passed through PRISMATIC ABERRATION. 41 a prism, is somewhat flattened, its section becomingslightly elliptical instead of circular; or, in otherwords, two rays which enter the eye in one meridianhave a more distant focus than two which enter itin another meridian. In Fig. 22, two rays proceed-ing from a point 0 enter the pupil of an eye in the. Fig. 22. vertical meridian. The inferior ray (a) is drawnso as to traverse the prism in the direction ofminimum deviation, and it therefore experiencesthe least possible deflection with which any raycan pass through a prism. Since the superior ray (J?) has a different angle ofincidence, it experiences a greater deflection, sothat on their emergence the two rays are lessdivergent than they were before. On this accountthey enter the eye as if they came from a point (T)a little further off than it would be if there were noprismatic astigmatism. Suppose now the prism were tilted a little round 42 PRISMS. its axis, so as to bring the superior ray, instead ofthe inferior, into the direction of minimum devia-tion. The conditions would then be reversed, forthe inferior ray would be most deflected, and thetwo rays would enter the pupil more divergent, andas if they came from a point nearer than they woulddo if there were no prismati
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