. Nature . I replaced the candle in the focus, arranged thecondenser of the spectroscope so that the white spotshould come upon the centre of the slit, and occupy one-third of it. The field of view was filled by the spectrumof the flame, but across the centre was a bright band oflight extending far into the violet, brightest in the blue,and showing nil t lie Frauenhofer lines distinctly, especiallyin the blue and violet. Unmistakably I was dealing withreflected light, and not with fluorescence. My thoughts atonce reverted to Prof. Tyndalls blue cloud. I knewnf two ways of producing an extremel
. Nature . I replaced the candle in the focus, arranged thecondenser of the spectroscope so that the white spotshould come upon the centre of the slit, and occupy one-third of it. The field of view was filled by the spectrumof the flame, but across the centre was a bright band oflight extending far into the violet, brightest in the blue,and showing nil t lie Frauenhofer lines distinctly, especiallyin the blue and violet. Unmistakably I was dealing withreflected light, and not with fluorescence. My thoughts atonce reverted to Prof. Tyndalls blue cloud. I knewnf two ways of producing an extremely fine precipitateshowing the same characteristic phenomena. I addeddilute hydrochloric acid to a weak solution of sodiumhyposulphite, but this preparation I found to be trouble-some from the rapidity with which it loses its opticalproperties, so I discarded it in favour of the following. Idiluted some French polish with about fifty times its bulkof methylated spirit, and added a few drops of the solution. Fig. i.—a, tumbler con polarising apparatus ; c .cardboard to screen offlight ; g, mirror ; H, sidedifferent azimuth may be 3 support, sheet of uperfluous light ; F, lens to concentrate^ thelirror in which the colour of the beam in a to a glass of water. The precipitate of lac resulting issufficiently fine for every purpose, and will remain insuspension for days. The light from the heliostat passingthrough this solution gives the same soft opalescent reflec-tion, with the same spectrum strongest in the blue andviolet, showing all the Frauenhofer lines distinctly, as itdoes upon the candle-flame. Experiment 3.—There is another special characteristicof matter in extremely fine division common to blue cloud and the above-mentioned solu-tions. Light reflected from it is completely polarised inthe plane at right angles to the line of incidence. I amin the habit of showing this by the following arrangement,which I believe to be new, and which is so simple thatany
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