MOUNT WILSON OBSERVATORY. 291 



corresponding to the three differences of pressure employed. The 

 sum of two of the differences should equal the remaining difference, 

 and in practice this is found to be so within 0.0001 a. Representative 

 lines belonging to various groups in the Mount Wilson classification 

 are under observation, the range of wave-length being X3900 to X6500. 

 The pressures are read on a mercurial manometer held constant within 

 1 cm. during an exposure; the values chosen include points at frequent 

 intervals within the range from zero to one atmosphere. At present 

 the observations are not complete enough to admit of satisfactory dis- 

 cussion, but there is no doubt that for the sensitive lines of groups 

 c and d the values of the pressure displacement obtained in this way 

 are markedly lower than those of former observers. The difference may 

 be ascribed to the lessened influence of pole-effect in our method of 

 observing. The program includes an examination of the effect of 

 change of atomic weight of the surrounding gas, using hydrogen and 

 helium instead of air, the study of modifications in the sources, and the 

 possible application of other types of interference spectroscopes to the 

 measurement of these minute displacements. 



ABSORPTION SPECTRA OF ELECTRICALLY EXPLODED WIRES, 



The work on absorption spectra by the method of electrically ex- 

 ploded wires has been continued by Mr. Anderson with the assistance 

 of Mr. Sinclair Smith. High-dispersion spectrograms of the iron 

 spectrum from X2600 to X6600 have been made with the 15-foot con- 

 cave grating. These show a wealth of detail, and complete study of 

 them will require some time. Perhaps one of the most interesting 

 facts brought to light is that the motion toward the observer of the 

 absorbing vapors surrounding the explosion causes all the absorption 

 lines to be displaced toward the violet, but by different amounts for 

 different classes of lines. In general the displacement is greater for 

 high than for low-temperature lines. 



For the further development and extension of the work, it is desirable 

 to have a condenser suitable for voltages up to 100,000 or more. Con- 

 densers of this type are not available at present, except by series con- 

 nection of low-voltage units. Investigation shows, however, that a 

 condenser of the required capacity made up of series units would be 

 exceedingly costly, and experience indicates very positively that it 

 would not function satisfactorily. Units capable of working at 100,000 

 volts therefore appear to be a necessity. A good deal of study has 

 been given to this problem, and it is hoped that a satisfactory solution 

 is in sight. 



MISCELLANEOUS INVESTIGATIONS. 



Messrs. Anderson and Smith have done some work on the effect of 

 passing the discharge of the large condenser through vacuum tubes. 

 This promises to be a powerful method of developing the enhanced 



