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16 transmitted through an aluminium sheet, are reflected by a polished plate of glass in conformity with the laws of regular reflection, and are diffused by a plate of ground glass.

These radiations traverse all the substances whose transparency I tested, with the exception of rock-salt 3 mms. thick (note 4), platinum 4 mms. thick, and water. A slip of cigarette paper, which is completely transparent when dry, becomes absolutely opaque when wetted with water. Figs. 2 and 3 are reproductions of the impression made in four seconds on a sensitive plate, without any photographic apparatus, before and after wetting the sheet of paper interposed between the lens and the spark. The photo-engraving, produced from a paper print, shows that in the first case the spark is notably brighter.

These photographic prints are produced by the spark influenced by the rays, and not by the rays themselves, these latter producing no appreciable photographic effect after an hour's exposure.

Amongst the bodies which are traversed, I