. The London, Edinburgh and Dublin philosophical magazine and journal of science. were certainly refracted; and consequentlywe proceeded to make measurements according to ordinaryoptical methods, using the apparatus shown in the (fig. 1) is a fairly powerful electric light produced by aGramme machine; 0 is a glass lens giving a parallel beam oflight, part of which passes through the slit, S, ^ inch wide,and falls on the edge of the ebonite prism P. F is a frameholding tissue-paper, which can be moved about P as centre,and which carries an index, I, pointing to the graduations onthe ci
. The London, Edinburgh and Dublin philosophical magazine and journal of science. were certainly refracted; and consequentlywe proceeded to make measurements according to ordinaryoptical methods, using the apparatus shown in the (fig. 1) is a fairly powerful electric light produced by aGramme machine; 0 is a glass lens giving a parallel beam oflight, part of which passes through the slit, S, ^ inch wide,and falls on the edge of the ebonite prism P. F is a frameholding tissue-paper, which can be moved about P as centre,and which carries an index, I, pointing to the graduations onthe circle, as seen in fig. 2. There was a fine vertical line inthe middle of the tissue-paper; andHGr, forming about one thirdof the paper, was well blackened. First this screen wasmoved into such a position that the edge of the prism threw ablack shadow which was bounded by the fine central line, andbetween that line and the blackened portion H G was a thinband of white light. In fact a narrow beam from the slit fell 198 On the Index of Refraction of 1. Fig. 2. LX- ?. on the edge of the prism; and half was stopped by the prism,the other half going on. The index-reading in this position wastaken; and now the screen was moved ronnduntil a red spec-trum was seen. At the least-refrangible end this spectrum termi-nated nearly abruptly, as the ordinary visible spectrum usuallydoes ; and this end was made to coincide with the central linein the screen, and the index-reading taken when, after movingthe prism itself, it was supposed that we had minimum devia-tion. The index-reading was also taken in the same way forthe most-refrangible end of the visible spectrum ; but as thisdid not die away at all abruptly, and as the whole spectrumwas very faint, the second set of measurements merely givesa rough idea of the amount of spectrum that was visible. Themean of a number of observations made by different observers,and the results of which were closely in accord, gave 1*66 asthe ind
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