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Rh small flame is by far the most convenient and precise of all processes for determining the position of the foci. Operating with the small spark is much harder, because the spark is rarely very regular.

I feel bound to reproduce, textually, here a passage in a letter which M. Gustave le Bon has done me the honour of writing—

"M. Gustave le Bon had indicated, as far back as seven years ago, that flames emit, independently of the radio-active emanations since observed by him, radiations of large wave-length, capable of traversing metals, and to which he had given the name of black light; but while assigning these a place intermediate between light and electricity, he had not exactly measured their wave-length, and the method he had employed to reveal their presence was very uncertain."

The method referred to was the photographic method. Personally, I have not been able to obtain any photographic effect of the rays I have studied (see p. 16).