The recent development of a technique whereby a metal is deposited by evaporation upon a mirror surface in a high vacuum now makes it possible to prepare surfaces of some of the elements not heretofore available. The most promising metallic surface, both in general reflecting power and in permanency, is that of aluminum. The work of Dr. John Strong of the California Institute of Technology, described elsewhere in this number, has made it possible to prepare mirror surfaces of this metal up to three feet in diameter, whence it is probable that aluminum will become eventually a substitute for silver. With the expectation of using an aluminized siderostat mirror in a re-determination of the solar energy-curve in the ultraviolet, the reflecting power of this material has been investigated in a preliminary way on Mount Wilson. For the spectral region to the violet of A0.45(i, photographic methods were employed with an iron arc as source and a 1 -meter concave grating spectrograph as the dispersion device. Timed exposures, variable sector, and variable source-distance gave sensibly the same results.1 For wave-lengths to the red of AO.45fx a radiometrie method was employed. A Hilger constant-deviation monochromator and thermopile was fed with sunlight, a very satisfactory source on Mount Wilson.2 The results, while not definitive, as they apply to only one pair of mirrors furnished by Dr. Strong, will be of interest to those planning to use evaporated aluminum surfaces. Table I shows the reflecting power of one aluminum surface at normal incidence, the reflecting power of a silvered mirror chemically deposited in the usual way, the sensitivity of the Eastman 40 photographic emulsion, the sensitivity of the eye, and the atmospheric transmission for Mount Wilson. The re-
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Edison Pettit (1934) studied this question.