. Refraction and how to refract : including sections on optics, retinoscopy, the fitting of spectacles and eye-glasses, nown as lateral inversion. This isbest understood by holding a printed page in front of aplane mirror, when the words or letters will read from rightto left. (See Fig. 3.) An observer facing a plane mirrorand raising his right hand, his image apparently raises theleft hand. Tilting a plane mirror gives an object the appearance of R r^ E F ^ 3 LEG 03J T ON HOIT Fig. 3.—Lateral Inversion. 14 REFRACTION AND HOW TO REFRACT. moving in the same direction to that in which the
. Refraction and how to refract : including sections on optics, retinoscopy, the fitting of spectacles and eye-glasses, nown as lateral inversion. This isbest understood by holding a printed page in front of aplane mirror, when the words or letters will read from rightto left. (See Fig. 3.) An observer facing a plane mirrorand raising his right hand, his image apparently raises theleft hand. Tilting a plane mirror gives an object the appearance of R r^ E F ^ 3 LEG 03J T ON HOIT Fig. 3.—Lateral Inversion. 14 REFRACTION AND HOW TO REFRACT. moving in the same direction to that in which the mirroris tilted. Spheric Mirrors.—A spheric mirror is a portion of areflecting spheric surface ; its center of curvature is thereforethe center of the sphere of which it is a part. Spheric mir-rors are of two kinds—concave and convex. Reflection from a Concave Mirror (Fig. 4).—Parallelrays are reflected from a concave mirror, and are broughtto a focus in front of it. This point is called the principalfocus (). The principal axis of a concave mirror is astraight line drawn from the center of the mirror to the. Fig. 4. center of curvature (i-i), and a secondary axis (2, 2,2, 2) is any other straight line passing from the mirror tothe center of curvature (). Rays which diverge fromany point beyond the principal focus are reflected con-vergently (G J). Rays which diverge from any point closerthan the principal focus are reflected divergently (V V). Images Formed by a Concave Mirror.—To find thepositi(Mi of an image as formed by a concave mirror, tworays may be used : one drawn from a given point on theobject to the mirror, and parallel to its principal axis, andreflected through the principal focus (, Pigs. 5 and 6);the other, the secondary axis, from the same point, passing OPTICS. 15 through the center of curvature. The place where thesecondary axis and the reflected ray or their projections in-tersect gives the position of the image. Unhke the planemirror, w
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