AN accurate knowledge of the transmission of quartz, in the infra-red, is essential in order to determine spectral ertergy curves. Quartz has a much larger dispersion than fluorite. Unfortunately, unlike fluorite, the absorption in quartz becomes quite marked at 2/* and is practically complete beyond 31". However, for the spectral region from the deep ultraviolet to 1.7~ in the infra-red, quartz has the remarkable property of complete transparency for the thicknesses (3 cm.) which are ordinarily used in optical investigations. This is illustrated in the present research, where it is shown that after eliminating the losses for reflection at the interface, quartz-air, the transmission is practically complete.
111 this investigation an especially prepared cylinder of clear quartz, 50 ram. in diafneter and 29.92 5 .mm. in thickness, cut perpendicular to the optic axis was used. The faces were plane to a fraction of a wave-length of light, and they were parallel to within I'. The source of energy-was a seasoned Nernst glower. operated on a storage battery. The rays from the glo,wer were made parallel by means of a 5 Β° cm. focal length silvered mirror, and from thence passed to a second mirror, 9 Β° cm. focal length, which brought the rays to focus upon the spectrometer slit. A blackened diaphragm having an opening of about 3 by 3-5 cm. was place d in the path of parallel rays. The quartz cylinder was mounted upon a suitabl e stage, sliding back and forth, close to and in the rear (i.e., between the diaphragnn and the second mirror) of the diaphragm. The ratio of the galvanometer deflection observed when the quartz plate was over the opening in the diaphragm, to the deflection caused by the rays passing through the diaphragm without obstruction by the quartz, gave the transmission, which, of course, includes the losses bv reflection from the two surfaces. Atmospheric disturbances were unusually small. * Communicated by the Bureau.