268 MITCHELL— THE FLASH SPECTRUM. 



The program of work on the flash specrum at the eclipse of 1918 

 was for the purpose of finding the heights of the various gases, and 

 also of extending our knowledge of the spectrum as far into the red 

 as possible. The Naval Observatory party located in Baker, 

 Oregon, had three concave gratings, the largest being of 2i>^ feet 

 radius and 15,000 lines in order to procure as large a dispersion as 

 possible; the second grating was of 10 feet radius and 15,000 lines, 

 while the third was of small dispersion to photograph into the ex- 

 treme red by the use of plates stained by dicyanin. Unfortunately, 

 thin clouds covered the sun during the whole of totality. The 

 clouds at the end of totality were exceedingly thin, but they were 

 sufificient to cut out most of the weaker lines from the spectra. At 

 no place in the country where photography of the flash spectrum 

 with great dispersion was attempted did clear skies prevail ; and con- 

 sequently the carefully prepared plans for 191 8 will have to be tried 

 again at the next available eclipse. 



McCoRMiCK Observatory, 

 University of Virginia. 



