324 Prof. Wood on the Achromatization of Monochromatic 
of the D lines did not provoke the fluorescence. Wiedemann 
and Schmidt observed a bright band in the fluorescence 
spectrum in the case of vapour confined in glass bulbs which 
appeared to coincide with the D lines. The same appearance 
J afterwards observed independently, but on continuing the 
study of the subject was forced to refer the existence of this 
band in the yellow to sodium in the flame which heated the 
bulb. This I have since found was a mistake, for on re- 
peating the experiments with exhausted glass bulbs, I have 
succeeded in stirring up a faint fluorescence with approxi- 
mately 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 experiments. 
made last year by Mr. Moore and myself was due to the 
fact that this yellow light was removed from the incident 
beam by the sodium vapour before the light-rays met at the 
focus. This work is still in progress, and the fluorescent 
-spectra given by the vapour when illuminated with mono- 
chromatic light of various wave-lengths have been photo- 
graphed. 
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. 
Johns Hopkins University, Baltimore, Md., 
May 28, 1904. 
XXXII. The Achromatization of Approximately Monochro- 
matic Interference Fringes by a Highly Dispersive Medium, 
and the consequent Increase in the Allowable Path-difference. 
By R. W. Woon, Professor of Experimental Physies, 
Johns Hopkins University *. 
HE results recorded in the present paper were, for the 
most part, obtained during the progress of an investi- 
gation of the dispersion of sodium vapour. As I have 
mentioned in the previous paper, the path-difference under 
* Communicated by the Author. 
