Elements of natural philosophy (Volume 2-3) . ey differ essentially withthe light employed: the light of the clouds, of the Moon,and of Venus, show them exactly as in the direct lightof the sun. The bright fixed stars give lines peculiarto themselves, as also do electric lights. The light offlames shows none, or at least onlv certain dark intervalsunder peculiar circumstances. These lines furnish themeans of measuring the refractive indices of differentmedia for different colors. Number of § 112. A question often proposed, as to the number of primary colors primary colors, can only be answered
Elements of natural philosophy (Volume 2-3) . ey differ essentially withthe light employed: the light of the clouds, of the Moon,and of Venus, show them exactly as in the direct lightof the sun. The bright fixed stars give lines peculiarto themselves, as also do electric lights. The light offlames shows none, or at least onlv certain dark intervalsunder peculiar circumstances. These lines furnish themeans of measuring the refractive indices of differentmedia for different colors. Number of § 112. A question often proposed, as to the number of primary colors primary colors, can only be answered with reference to considered; x . the sense in which it is asked. If it be meant to applyto the number of tints distinguishable in the spectrum,this will be a matter of individual judgment to differenteyes. Newton distinguished seven, Sir John IIersciieleight, Sir David Brewster three ; but perhaps most ob-servers would admit that it is impossible to fix on anydefinite number, since the light appears to go through ELEMENTS OF OPTICS. 297. every possible shade of color, from the deep red to faint odors of the . -i -, t T£» -, . -. ., ,. solar spectrum violet or lavender grey. It we understand the question resolvcd int0as applying to the number of definite points at each of eight a wave of different length occurs, their numbermust be considered as infinite. These waves resolvethemselves into eight classes, distinguished by the colorthey excite in the mind, the same color of different shadesbeing produced by waves whose lengths vary betweencertain limits. §113. To find the index of refraction for any one of To find the these different CO- refractive index Yirr 80 for any color; lors, let A be a re-fracting prism,madeof any transparentmedium; m ?i, a gra-duated circle, to thecentre of which asmall telescope isattached in such a manner that its line of colli-mation shall move in a plane parallel to that of the gra-duated circle, which is held in a position at righ
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