Ocular refraction and the shadow test . d II, figure 85 illustrates the effect. Now rotate the lens 90, so that the vertical straight line also ap-pears unbroken and passing through the other two points; move thelens to the right and left as before and the line as seen through it willnot appear to move. No refraction occurs in this meridian and thisindicates the axis. Figure 86 illustrates this experiment, no matter ifthe cylinder be convex or concave, so long as the dotted line remains LENSES. 83 at right angles to the vertical line, no distortion or displacement takesplace. The effect optica
Ocular refraction and the shadow test . d II, figure 85 illustrates the effect. Now rotate the lens 90, so that the vertical straight line also ap-pears unbroken and passing through the other two points; move thelens to the right and left as before and the line as seen through it willnot appear to move. No refraction occurs in this meridian and thisindicates the axis. Figure 86 illustrates this experiment, no matter ifthe cylinder be convex or concave, so long as the dotted line remains LENSES. 83 at right angles to the vertical line, no distortion or displacement takesplace. The effect optically is that of piano glass. These experimentsdemonstrate the fact that the power of a cylinder is at right angles toits axis. The characteristics of a piano cylinder lens may be summarizedin a few words as follows:— First:—Upon rotating it, objects seen through it appear distorted. Second :—In two positions a vertical straight line appears throughit unbroken, in each position the line is parallel to one of its A vertical straightangles jgh various portions of a piano-cylinder lens its axis at rightThe cylinder may be either convex or 1 Third:—Motion occurs at right angles to one of these meridianswhich is the axis. Fourth:—If the motion of the object seen through it is againstthat of the lens, the cylinder is convex; if it is concave the motionwill be with that of the lens If a vertical straight line be viewed through a prism whose baseline is at right angles to it, the line will appear unbroken, no matterthrough what portion of the prism it is seen; see II, Figure 87. Ifthe base line-of the prism be parallel to the line it will appear broken,that portion of the line seen through the prism being displaced towardthe apex of the prism; see I, Figure 87. The amount of displacement OCULAR R E F A C T I O N . is the same regardless of the portion of the prism through which theline is seen. If a prism be combined with a spherical, cylinder or sphero-cyl
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