Elements of natural philosophy (Volume 2-3) . Appearancesaccounted for. ELEMENTS OF OPTICS. 211 APPLICATION TO THE DEVIATION OF LIGHTBY SPHERICAL REFLECTORS. § 51. In reflexion, we have only to consider one <le- E(i»ation , t^ • tit • applicable to a viatmg surface. Equation (20) applies here by making spherical concave m = - 1, Which reduces it tO, reflector; 1_ f 2^r 1 7 (43) But two cases can arise, and these are distinguished bythe sign of the radius. The reflector may be concavetowards incident light, in which case r will be positive,or it may be convex towards the same direction, when
Elements of natural philosophy (Volume 2-3) . Appearancesaccounted for. ELEMENTS OF OPTICS. 211 APPLICATION TO THE DEVIATION OF LIGHTBY SPHERICAL REFLECTORS. § 51. In reflexion, we have only to consider one <le- E(i»ation , t^ • tit • applicable to a viatmg surface. Equation (20) applies here by making spherical concave m = - 1, Which reduces it tO, reflector; 1_ f 2^r 1 7 (43) But two cases can arise, and these are distinguished bythe sign of the radius. The reflector may be concavetowards incident light, in which case r will be positive,or it may be convex towards the same direction, whenr will be negative. Equation (-43) relates to the firstcase, which will now be discussed. If the incident rays be parallel, —- = 0, and / Incident raysparallel; / or. / = 2 r r. = f, 2 Ffe Principal focaldistance; Hence the principal focal distance is equal to half radius,and Equation (43), reduces to 1_ / 1 F 1 f /A4\ Equation for^ / discussion; Now, this Equation is only concerned with the re-flected wave, and if this wave be concentrated at allafter deviation, it must be upon that part of the axison the side of the incident light, and hence /, for a 212 NATURAL PHILOSOPHY. Real radiants reaj focus must be positive, and for a virtual focus ne- beyond tho x principal focus; gatlVe. and the vergency of the deviated rays will be positive;that is, a wave proceeding from a point in front of thereflector between the principal focus and infinity will,after deviation, be concentrated into some other point infront after reflexion. Real radiants When -_ < —, or/e negative; in other words, a waveproceeding from a point on the axis between the vertexand principal focus, will never be conc
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