396 PROCEEDINGS OF THE AMEUICAN ACADEMY. 



coincide with the D Hues. The same appearance I afterwards observed 

 independently, but on continuing the study of the subject was forced to 

 refer the existence of this baud in the yellow to sodium in the flame 

 which heated the bulb. This I have since found was a mistake, for on 

 repeating the experiments with exhausted glass bulbs I have succeeded 

 in stirring up a faint fluorescence with approximately monochromatic 

 light from the illuminator already referred to, of wave-length equal to 

 that of the D lines. The failure to observe it in the case of the experi- 

 ments made last year by Mr. Moore and myself was due to the fact 

 that this yellow light was i-emoved from the incident beam by the 

 sodium vapor before the light rays met at the focus. This work is 

 still in progress, and the fluorescent spectra given by the vapor when 

 illuminated with monochromatic light of various wave-lengths have been 

 photographed. 



The investigations recorded in the present paper have been made 

 possible by a very generous grant from the Rumford Fund, and I wish to 

 express to the trustees of the fund my appreciation of and thanks for the 

 aid which I have received. 



I wish also to express my appreciation of the very faithful work done 

 by my Assistant Mr. A. H. Pfund, who has worked with me and made 

 many valuable suggestions, and my thanks to the board of trustees of the 

 Carnegie Institution for the means placed at my disposal, by which I 

 have been able to secure his services. 



