. The Astrophysical journal. ess than the probableerror of measurement. It is worth noting that there is a Zn linewhose wave-length is about greater, but this is a pair line,strong in the spark spectrum and very weak in the arc in air. Thisis just opposite to the behavior of the Mg line, so that they almost NOTES ON CERTAIN ULTRA-VIOLET SPECTRA 237 never occur together. Eder and Valenta give a Cd line here also,but it seems likely that this was due to Zn. Photographs with the vacuum arc, and with the Mg vaporlamp also, show the next line of this same Mg series, for whichthe value X
. The Astrophysical journal. ess than the probableerror of measurement. It is worth noting that there is a Zn linewhose wave-length is about greater, but this is a pair line,strong in the spark spectrum and very weak in the arc in air. Thisis just opposite to the behavior of the Mg line, so that they almost NOTES ON CERTAIN ULTRA-VIOLET SPECTRA 237 never occur together. Eder and Valenta give a Cd line here also,but it seems likely that this was due to Zn. Photographs with the vacuum arc, and with the Mg vaporlamp also, show the next line of this same Mg series, for whichthe value X is obtained. This agrees with the value givenby Lyman, who was the first to measure this line. The sparkspectrum of ]\Ig was photographed in this region, but showed nonew lines. CALCIUM The measurements of the Ca spectrum in the region of X 2000given in an earlier paper have since been improved by gettingbetter photographs. One of these is of sufficient interest to beworth reproducing (Fig. ib). It was taken from an arc be-. FiG. I.—Region of X 2000 a. Mg arc in air, X 2026 only. b. Ca arc in air, showing principal series of single lines. tween two pieces of metalhc Ca, of which the positive one was onthe point of melting in air. A short-focus concave grating and aSchumann plate were used. The reproduction, enlarged fourfold,shows the reversed lines of the single-hne principal series (called SLibefore), hea\dly overexposed. The emitted light from these lines inthe case of the last four fuses together into a continuous background,upon which the reversals stand out as absorption lines. The char-acter of these lines in this photograph is strong evidence for classi-fying them together as a principal series. The first two lines of this Astrophysical Journal, 32, 153, 1910. 238 F. A. SAUXDERS series are the flame line X 4226 and a line at X Rowland(air), which is peculiar in two ways. First, it was discovered byRowland but was overlooked by later observers, and has not yet
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