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60 numbers deduced from observation. Although I cannot with certainty indicate the degree of approximation of the results, I believe, nevertheless, that the relative errors do not exceed 4 per cent.

The wave-lengths of "N" rays are much smaller than those of light. This is contrary to what I had imagined for a moment, and contrary to the determinations which M. Sagnac thought he had deduced from the position of the multiple images of a source, obtained with a quartz lens, images attributed by him to diffraction. I had previously observed that while polished mica lets "N" rays pass, roughened mica stops them, and also that whereas polished glass reflects them regularly, ground glass diffuses them. These facts were already an indication that "N" rays could not have large wave-lengths. If we desire to study the transparency of a body, we must take care that the surface is well polished. Thus I had at first classed rock-salt amongst opaque substances, because the specimen I used, having been sawn from a large block, had remained unpolished; in reality, rock-salt is transparent.