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12 no longer produces any fluorescence, and is itself absolutely invisible in the dark. They are fainter, however, in this case. They can also be produced continuously by means of an electric machine giving a spark a few millimetres in length.

At first I had attributed to Röntgen rays the polarization which in reality belongs to the new rays, a confusion which it was impossible to avoid before having observed the refraction, and it was only after making this observation that I could with certainty conclude that I was not dealing with Röntgen rays, but with a new species of light.

It is interesting to collate these remarks with the view expressed by M. Henri Becquerel, that in certain of his experiments "manifestations identical with those giving refraction and total reflection of light may have been produced by luminous rays which had traversed aluminium" (see Comptes Rendus, t. xxxii., March 25, 1901, p. 739).