Elements of natural philosophy (Volume 2-3) . dent ray and the same rayafter twoOr reflexions; SSD = is, the angle made by the incident ray and the 18S NATURAL PHILOSOPHY. Equal to double same ray after two reflexions, is equal to double the an-the angle made gje 0f t]ie reflectors. It follows, therefore, that if the angle of the reflectors be increased or diminished by giv-ing motion to one of the reflectors, the angular velocityof the reflected ray will be double that of the of This is the princi])le upon which reflecting instrumentstins principle. £or ^ measure
Elements of natural philosophy (Volume 2-3) . dent ray and the same rayafter twoOr reflexions; SSD = is, the angle made by the incident ray and the 18S NATURAL PHILOSOPHY. Equal to double same ray after two reflexions, is equal to double the an-the angle made gje 0f t]ie reflectors. It follows, therefore, that if the angle of the reflectors be increased or diminished by giv-ing motion to one of the reflectors, the angular velocityof the reflected ray will be double that of the of This is the princi])le upon which reflecting instrumentstins principle. £or ^ measurement of angles are constructed. DEVIATION OF LIGHT AT SPHERICAL SUPwFACES. Deviation of § 29. Let MD 0 JV, be a section of a spherical surface light at sphericalsurfaces; separating two me-dia of different den-sities, as air andglass, having its cen-tre at C, on the line Illustration and 0 C, wllicil will be called the axis ofthe deviating sur-face ; FD a ray oflight, incident at D, Fig. 15. <?^ notation Vertex. Rule first; Rule and D S, the direction of this ray after deviation, whichbeing produced back will intersect the axis at F!. Thepoint 0, where the axis meets the surface, is called thevertex, which will, for the present, be taken as the FD, u; F D, vl; CD,r; OF,f; 0 F, f ;and the angle 0 CD, 0. Now, distances estimated in the direction of wave pro-pagation, from any origin whatever, are alioays negative ;those estimated in the contrary direction^ positive. And, when light is incident on a concave surface, theradius of curvature is cdwayspositive / when incident ona convex surface, negative. In the triangle CDF, wTe have the relation, sin <p f — r sin 0 u ELEMENTS OF OPTICS. 189 and in the triangle CDF, sin & u sin 9 f—r Equations fromthe figure; These combined with sin 9 = m sin 9 , Combined withthe generalequation ofdeviation ; give mu.(f-r) = u(f-r) .... (17) The first of these triangles will also give, u 2 = (/ — r)2 + r2 + 2 (/ —
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