Skiascopy and its practical application to the study of refraction . the light ismost accurately focused on the retina in the direction thatthe band should take. And, in the direction at right anglesto the band, the focusing is quite incomplete, so that thediffusion at what should be the sides of the band partly orentirely neutralizes the effect produced by the greater mag-nification of the retina in the direction of the band, which,otherwise, would cause the band-like appearance. In order to bring out this band-like appearance, it isnecessary to make the focusing from side to side of the band
Skiascopy and its practical application to the study of refraction . the light ismost accurately focused on the retina in the direction thatthe band should take. And, in the direction at right anglesto the band, the focusing is quite incomplete, so that thediffusion at what should be the sides of the band partly orentirely neutralizes the effect produced by the greater mag-nification of the retina in the direction of the band, which,otherwise, would cause the band-like appearance. In order to bring out this band-like appearance, it isnecessary to make the focusing from side to side of the bandas perfect as possible. And, to secure the perfect focusingin the principal meridian at right angles to the one inwhich the band is sought, the immediate source of lightmust be brought to the point of reversal for that otherprincipal meridian. Ihe band-like appearance is most per-fectly developed when the observers eye is at the point of reversalfor one principal meridian, and the immediate source of light atthe point of reversal for the other principal Fig. figure 9, the solid lines represent the vertical merid-ian of an astigmatic eye; and the rays emerging, so turnedin that meridian as to give the point of reversal at V. Thebroken lines represent the less curved horizontal meridianof the cornea, and the rays so turned in that meridian as togive a point of reversal at H. The dotted lines represent a THE BAND-LIKE APPEARANCE. 49 plane mirror, P P, with the eye of the observer at V) andthe light L pushed off from the mirror, so that the raysenter the eye as though they came from H) and are per-fectly focused on the retina in the horizontal meridian, ren-dering most distinct the appearance of a vertical band. For illustration, suppose a case [which the student willdo well to reproduce for actual study, either in the artificialeye or by lenses placed before the living eye] having com-pound myopic astigmatism, the vertical meridian of thecornea being 2 D. myopic an
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Keywords: ., bookcentury1800, bookdecade1890, booksubjectretinos, bookyear1896
