The outlines of physics: an elementary text-book . motionof the earth, the instrument is called a heliostat. REFLECTION AND REFRACTION 373 a given point in the object and the image of that point isbisected by the mirror, and is perpendicular to the surfaceof the same. To an observer looking towards a planemirror the image appears in the direction of the ray fromthe face of the mirror to his eye. The position of theimage is found by turning the reflected ray r (Fig. 344)until it coincides in direction with the incident ray i; the position r behind the mirror. The sum of thedistances i
The outlines of physics: an elementary text-book . motionof the earth, the instrument is called a heliostat. REFLECTION AND REFRACTION 373 a given point in the object and the image of that point isbisected by the mirror, and is perpendicular to the surfaceof the same. To an observer looking towards a planemirror the image appears in the direction of the ray fromthe face of the mirror to his eye. The position of theimage is found by turning the reflected ray r (Fig. 344)until it coincides in direction with the incident ray i; the position r behind the mirror. The sum of thedistances i + r will be the same as i + r. The image willtherefore be as far behind the mirror as the object is infront of it. 340. Concave and Convex Mirrors.—In the case of curvedsurfaces, although the law of reflection is the same/or eachpoint of the surface, as in the case of plane mirrors, theresults produced are very different. The most importantcase is that of the concave mirror. This has a surfacewhich is a portion of the inner surface of a Fig. 345. (1) Consider the rays of light from a point a in frontof the mirror. Those which reach the surface of themirror are reflected, each one in accordance with the lawof reflection given in the foregoing article. As will beseen from Fig. 845, all these rays comQ together at a 374 THE OUTLINES OF PHYSICS single point h in front of tlie mirror. If, on the otherhand, the source of light were situated at h, rays emanat-ing from it, and reaching the mirror, would all be reflectedto a. The points a and b are called conjugate foci. (2) If the source of light be situated at the center ofthe mirror 0, the angles of incidence and reflection arereduced to zero. Since all rays strike the surface of themirror normally, they will be reflected directly back upontheir course, and will be brought to a focus at 0. (3) Consider the rays from a point F halfway betweenthe center of the mirror and its surface (Fig. 346). The reflected rays will be eve
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