Elements of natural philosophy (Volume 2-3) . Colors of thickplates; How they may-be exhibited; 292 NATURAL PHILOSOPHY. Facts withregard to thesecolors; How Illustration and cave glaSS reflector M N, With COIl-explanation; centric sUrfaCes, the back being sil-vered, and its centre of curvaturesituated at the aperture, there willbe formed upon the screen aboutthe aperture a series of colored rings,or in luminous waves of a singlelength, alternate bright and dark cir-cles. These become faint and disap-pear if the distance of the screenbe increased or diminished beyond a small differen

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Elements of natural philosophy (Volume 2-3) . Colors of thickplates; How they may-be exhibited; 292 NATURAL PHILOSOPHY. Facts withregard to thesecolors; How Illustration and cave glaSS reflector M N, With COIl-explanation; centric sUrfaCes, the back being sil-vered, and its centre of curvaturesituated at the aperture, there willbe formed upon the screen aboutthe aperture a series of colored rings,or in luminous waves of a singlelength, alternate bright and dark cir-cles. These become faint and disap-pear if the distance of the screenbe increased or diminished beyond a small differencefrom its original position. They diminish in diameteras the glass is thicker. They arise from the interferenceof the waves reflected from the back surface with thosereflected from the front. Denote by y, the radius A D, of one of the rings, eitherdark or bright; by t, the thickness C E, of the reflector;and by r, the radius A C. The equivalent interval to t, inair will be m t, in which m denotes the relative index ofrefraction of air and glass. Then Notation; Equation; CD= ^T2 + y2 ~ r + y- 2r neglecting the 4th and higher powers of like manner, Another;

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