MOUNT WILSON OBSERVATORY. 207 



XX4553-75, belonging to the second stage of ionization, was found to be just 

 beyond the reach of this excitation. Measurements of X4103 on photographs 

 of the arc in air agree closely with the tube-arc value. 



Lines which are difficult to measure with accuracy in the arc and spark, 

 on account of diffuseness, dissymmetry, or tendency to reverse, may be 

 photographed as sharp lines in the vacuum furnace by a proper selection of 

 temperature. The addition of iron to the furnace supplies standard lines. 

 Lines of the sharp and diffuse series of magnesium and X4227 of calcium 

 have been photographed in this way for the use of St. John in a comparison 

 of solar and laboratory wave-lengths. 



King has also made a number of furnace and arc spectrograms of calcium, 

 strontium, and barium in the ultra-violet to provide the material for studies 

 of the series relationships in these spectra. 



REGULARITIES AND GROUPS IN LABORATORY SPECTRA. 



Dr. Russell has devoted considerable attention to the study of groups 

 of lines in spectra obtained from laboratory sources. Several groups in the 

 spectra of strontium and barium have been identified as "combinations" 

 between recognized spectroscopic terms and new terms, and further evidence 

 has been obtained in support of Bohr's view that the corresponding quantum 

 changes involve simultaneous alterations in the orbits of two electrons. 



An investigation of the spectrum of titanium, undertaken with the aid of 

 data furnished by King and Babcock, has revealed striking regularities. More 

 than 250 lines have so far been classified, including all the strongest and a large 

 majority of those which appear at low temperatures. The spectroscopic 

 terms so far discovered are triple or five fold (perhaps analogous to the single 

 and triple terms in calcium), and their combinations give rise to ''multiplets," 

 or groups of from 6 to 13 lines. The strongest members of these groups 

 account for the conspicuous sets of 3, or occasionally 5, lines which are so 

 prominent a feature of the titanium spectrum. Since many of these lines, 

 originating from different levels, are in the region included by ordinary 

 stellar spectrograms, they are likely to be of considerable astrophysical 

 importance. 



No series of the ordinary type have yet been found in titanium, and the 

 ionization potential can not be determined. The principal resonance poten- 

 tial appears, however, to be 2.38 volts, corresponding to the strong lines 

 XX5173, 5192, 5210, with a weaker one, corresponding probably to an "outer 

 system" combination, at 2.27 volts (XX5426, 5460, etc.). There may be one 

 at 0.80 volt, corresponding to a transition between the' lowest spectroscopic 

 term and the next, but observations in infra-red are required to decide this. 



Dr. and Mrs, Kiess have discovered 10 of these multiplets (Proc. Wash. 

 Acad. Sci., 13, 220, July 1923), but the arrangement of the lines differs in 

 several cases. The present investigation was well under way before it was 

 known that they were at work on the problem. 



ELECTRICALLY EXPLODED WIRES. 



The work with electrically exploded wires has been continued by Anderson, 



with the assistance of Sinclair Smith. An important improvement is the use 



of a rotating octagonal mirror in place of the single plane mirror, with the 



result that every spark or explosion projected on the slit is recorded on the 



