Ocular refraction and the shadow test . a double purpose, by means of it we canidentify a cylinder and locate its principal meridians. The next pointis to determine which of these is the principal meridian and which the 82 OCULAR REFRACTION. axis. Rotate the plane cylinder to a position in which the verticalstraight line appears through it unbroken; by reference to II, figure48 the effect desired will be seen, the line passing through two of thepoints marked upon the lens. With the lens in this position repeatexperiment illustrated by figure 4^5. If the motion of the lens to theright and left
Ocular refraction and the shadow test . a double purpose, by means of it we canidentify a cylinder and locate its principal meridians. The next pointis to determine which of these is the principal meridian and which the 82 OCULAR REFRACTION. axis. Rotate the plane cylinder to a position in which the verticalstraight line appears through it unbroken; by reference to II, figure48 the effect desired will be seen, the line passing through two of thepoints marked upon the lens. With the lens in this position repeatexperiment illustrated by figure 4^5. If the motion of the lens to theright and left causes the portion of the lens seen through it to move,breaking and causing its displacement, refraction must take place inthis meridian. Knowing that no refraction occurs in the axis of acylinder, this meridian must be the principal meridian. Connect thetwo points marked up )n the lens that are in the vertical plane, whenthe lens is in this position, with a line of dots marked with the indiaink, see figure 85, tins zvill be the Displacement If the motion of the object seen through the lens be opposite oragainst that of the lens, the cylinder is convex, the appearance createdis shown by 1, figure 85. If the motion of the object viewed throughthe lens be the same as that of the lens, or with it, the cylinder is aconcave and 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
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