The Reflective Power of Some Substances in the Ultraviolet. P. R. GLEASON. (Proc. Nut. Acad. Sciences, July, 1929.) For the range of wave-lengths from 585 A. to 1850 A. there have been no data on reflecting powers for individual frequencies.
In this investigation light from discharge tubes containing, hydrogen, helium, oxygen or nitrogen was analyzed in a vacuum spectrograph by reflection from a concave speculum grating.
Light of a particular wave-length was made to fall on a photographic plate either directly or after reflection from the substance whose reflecting power was to be measured.
" In practice the exposure times of the direct and reflected spectra were adjusted so as to give the same opacity of the spectral line image in both cases. If the source remained constant in intensity throughout, the reflecting power was given by the ratio of the two exposure times." The following substances were worked with,-aluminium, beryllium, brass, chromium, gold, nickel, platinum, silicon, silver, speculum steel, stellite, fluorite, glass and quartz For the wave-length 585 A. freshly sputtered platinum has the largest reflecting power, 18.6 per cent., and this it keeps for many months.
In its fresh state its reflecting power is more than double that of any common substance used for gratings.
I' Nickel, crystalline quartz and gold are the only other substances reflecting more than IO per cent. at 585 A.
Chromium and silicon are both superior at the longer wave-lengths but do not hold up to a high value at the shorter wave-lengths."
Quartz, glass and fluorite have a larger reflecting power near 1200 A. than at 585 or 18.50 A. Beryllium for 1640 A. shows a reflecting power of only I per cent. This may mean that its index of refraction for this wave-length is unity.
G. F. S.