76 REPORTS ON INVESTIGATIONS AND PROJECTS. 



IvABORATORY INVESTIGATIONS. 



A brief description of the spectroscopic laboratory, constructed last 

 summer on Mount Wilson, was given in the last annual report. A more 

 complete account may be found in Contributions from the Solar Observa- 

 tory, No. lo. The arrangement of the various light sources on an annular 

 pier, in such a way that the image of any one of them can be brought 

 upon the slit of the spectroscope, by setting a mirror at the center of the 

 pier at the proper angle, has proved very convenient in practice. The 

 study of spectra, which was begun by Mr. Gale immediately after he 

 joined our staff last spring, has necessarily involved a comparison of a 

 great number of light sources. As already explained, the hypothesis 

 which served as a guide in our attempt to account for the strengthened 

 lines and other phenomena of spot spectra assumes that their tempera- 

 ture is the principal variable concerned. As no electric furnace was 

 then available, the work was begun with the aid of the ingenious form of 

 synchronous arc used by Professor Crew, which permits the spectrum 

 of a low voltage alternating discharge to be photographed at any desired 

 phase from o to 90 degrees. The changes in the spectra recorded at de- 

 creasing phase angles correspond to those produced by decreasing tem- 

 peratures, so that the device affords a simple means of testing the effect 

 of temperature variations. The exposures corresponding to small phase 

 angles are necessarily very long with this apparatus, and it therefore 

 occurred to Mr. Gale that a reduction of the current in an ordinary 

 carbon arc might accomplish the same purpose, and give brighter spectra. 

 A comparison of the results given by a 30-ampere and a 2-ampere arc 

 justified this view, and served as the most practical means at our dis- 

 posal of producing a comparatively low temperature in the case of such 

 metals as titanium and vanadium. The results obtained, and their bear- 

 ing upon the spectra of sun-spots, have already been mentioned. The 

 important fact was also discovered that those lines which are "enhanced" 

 in the electric spark are very appreciably weakened in passing from the 

 30-ampere to the 2-ampere arc. The importance of this fact arises from 

 the diverse opinions which have been expressed regarding the cause of 

 the enhanced lines, and their production under different conditions. 

 Special investigations of this subject have led many spectroscopists to 

 believe that other variables may be quite as important as temperature 

 in their effect upon the intensity of the enhanced lines. Crew, for ex- 

 ample, has expressed the opinion that the essential cause of the produc- 

 tion of enhanced lines in arc spectra is a rapidly changing electromotive 

 force. Hartmann, on the other hand, in his work on the arc in water, 

 ascribed enhanced lines observed under these conditions to the effect of 

 hydrogen gas produced by the decomposition of the water. It accord- 



