radiations foreign to "N" rays. In front of those screens, and at a distance of 14 cms. from the lamp filament, a large screen of wet cardboard is arranged, in which a slit has been cut 5 mms. wide and 3.5 cms. high, exactly opposite the lamp filament. In this way I obtain a well-defined pencil of "N" rays; this pencil is received on an aluminium prism whose refractive angle is 27° 15′, placed so that one of its faces is normal to the incident pencil.
It is, then, possible to prove that from the other refractive face of the prism several pencils of "N" rays, horizontally dispersed, emerge. For this purpose a slit 1 mm. broad and 1 cm. high, cut in a sheet of cardboard, is filled with calcium sulphide rendered phosphorescent; by displacing this slit, the position of the dispersed pencils is determined without difficulty, and the deviations being known, their refractive indices are deduced. This is the method of Descartes. I thus established the existence of "N" radiations, whose indices are respectively 1.04, 1.19, 1.29, 1.36, 1.40, 1.48, 1.68, 1.85. In order to measure with more exactness the first two indices, I made use of another aluminium prism
