252 PROCEEDINGS OF THE AMERICAN ACADEMY. 



higher order, but by the presence of a number of spectra of lower 

 dispersion. 



2. The number and dispersion of these false spectra obey no simple 

 law, but present the appearance of a reproduction of some real group or 

 groups distributed at random. 



3. The number, dispersion, and clearness of these spectra differ with 

 different gratings. 



1. The lines due to the false spectra are most clearly visible in the 

 extreme ultra-violet, owing to the absence of strong real lines, and are 

 so pronounced as to screen any feeble real line that may occur in this 

 region. 



Jefferson Physical Laboratory, Harvard University, 

 Cambridge, Mass., October, 1000. 



