MOUNT WILSON OBSERVATORY. 287 



The furnace experiments show the enhanced lines to be of a type inter- 

 mediate between those of titanium and the H and K lines of calcium, 

 requiring only moderate excitation, some of them showing in the fur- 

 nace at 2250°. This was indicated by Fowler's work, but the furnace 

 experiments have yielded much additional information, as many lines 

 of this type are faint in the arc and can be advantageously observed 

 only with the furnace. The band spectrum of scandium was found 

 to be due probably to the oxide. 



The low-temperature lines undergo a wide magnetic separation in 

 sun-spots, so that when laboratory data are available these lines will 

 be useful in the study of spot fields. 



Absorption Spectra. 



The use of the furnace as a source for absorption spectra, the in- 

 candescent background being given by a plug at the center of the tube, 

 has been continued. The main objects were the production of ab- 

 sorption spectra comparable in richness with the emission spectrum, 

 and the observation of the relative reversibility of different classes 

 of lines. Higher temperatures were employed than formerly, plug 

 temperatures above 3200° C. sometimes being used. With iron, 

 practically the whole furnace spectrum was thus obtained in absorp- 

 tion as far as X6700. The iron lines to the red of X5500 are difficult to 

 reverse in laboratory sources. Hitherto they have been obtained as 

 absorption lines only in the explosion spectrum. 



A detailed comparison was made of the iron spectrum in emission 

 and absorption at different temperatures. \^Tien supplemented by 

 experiments now in preparation, these results will show clearly the 

 connection between the susceptibility of a certain type of line to re- 

 versal and its general laboratory behavior. The high temperatures 

 which may now be used permit the study of lines in absorption which 

 in the usual sources show no tendency to reverse. The development 

 in absorption of the high-temperature ar? lines (Class III) is of special 

 interest. An extension into the ultra-violet has also been obtained, 

 the absorption spectrum of iron having been carried to X2448. 



Absorption spectra of the alkali metals, including the subordinate 

 series of sodium, potassium, and caesium, have also been produced in 

 the furnace. The principal series lines of these substances had pre- 

 viously been observed in absorption, but not those of the subordinate 

 series. The furnace shows, however, there is no difficulty in producing 

 these series beyond that of using a sufficiently high temperature in 

 vacuum. 



The sliarpness of these furnace absorption lines, as compared with 

 their diffuse and often very unsynmietrical structure in the arc, 

 showed that unproved wave-lengths could be secured and, conse- 

 quently, that the values of the series constants could be improved 



