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I have recently proved that the majority of artificial sources of light and heat emit radiations which are able to traverse metals and a great number of bodies, opaque in regard to the spectral radiations hitherto known (see p. 18). It was desirable to ascertain whether radiations analogous to the former—which, for brevity, I call "N" rays—are also emitted by the sun.

As I have shown, "N" rays act on phosphorescent substances by heightening or stimulating the pre-existing phosphorescence, an action similar to that of red and infra-red rays discovered by Edmond Becquerel (see p. 74). I utilized this phenomenon to find out whether the sun sends us "N" rays.

A completely enclosed dark room has one window exposed to the sun. This is shut by interior, opaque panels of oak, 15 mms. thick. Behind one of these panels, at any distance—1 metre, for instance—a thin glass tube is