Ocular refraction and the shadow test . he LENSES. 69 reading of the axis indicated by the trial frame is made by the observ-er as he faces his patient, his line of vision thus meets the outer oranterior surface of the lens. In looking through a lens at the Pro-tractor scale the line of vision meets the inner or posterior surfaceof the lens. This explains the apparent difference in the positions ofthe numbers of the trial frame scale and the protractor scale. The vertical and horizontal axes read the same on both, but all others seemreversed, thus; Axis 45 degrees on the trial frame appears to
Ocular refraction and the shadow test . he LENSES. 69 reading of the axis indicated by the trial frame is made by the observ-er as he faces his patient, his line of vision thus meets the outer oranterior surface of the lens. In looking through a lens at the Pro-tractor scale the line of vision meets the inner or posterior surfaceof the lens. This explains the apparent difference in the positions ofthe numbers of the trial frame scale and the protractor scale. The vertical and horizontal axes read the same on both, but all others seemreversed, thus; Axis 45 degrees on the trial frame appears to be axis35 degrees on the protractor. A spherical and a cylinder power may be combined in one lens, ineffect it is a combination of a piano-spherical and a piano-cylinder, asthe spherical curvature is ground upon one surface and the cylindricalcurvature upon the other. The spherical and cylindrical curvaturemay be ground upon the same surface; such lenses are called Torusor Toric. They have a noticeable curvature, similar to a coquille. Figure 72. Plano-convex splierical and plano-convex cylinder combined; showing tlie power of a genericcompound in its principal meridians. form, an J within certain limitations are a decided improvement overthe other forms in both appearance and satisfaction to the give a wider field of vision and are free from annoying thickness of the lens has pfactically little effect upon the opticalvalue, that is dependent upon the curvature of the surfaces. Let I, figure 72, represent a piano convex spherical of 2,00 D.; II,represents a plano-convex cylinder of D.; Ill, represents the cylin-der in combination with the spherical, its axis at 90°. The formula forthis compound will + D. S. 3 + i-o^ ^- Cyl. ax. 90°and it will be classed as a generic. In the meridian parallel to theaxis of the cylinder the power of the combined curvatures will be that 70 OCULAR REFRACTION. of the spherical alone, while in the meridian
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