MOUNT WILSON OBSERVATORY. 217 



The use of the method of the vacuum spark has widened very greatly 

 the field of study of the spectra of the elements. The result found in the 

 case of calcium, a spectrum rich in lines, very few of which can be identified 

 with known lines of this element, leads to the conclusion that a state of ioni- 

 zation is attained which is decidedly beyond that reached in ordinary 

 sources. Numerous other elements are now under investigation in the 

 laboratory by this method, and the results are certain to prove of physical 

 interest and very possibly to have astrophysical applications. 



The determination of the wave-lengths of spectral lines in the electric 

 furnace at ordinary pressures and in vacuo, interferometer observations of 

 wave-length, radiometric measurements of the energy of various light- 

 sources, and studies of the width and intensities of lines with the registering 

 microphotometer form a portion of the other investigations in progress in the 

 physical laboratory. 



