EXPERIMENTS WITH INVISIBLE LIGHT—WoopD. 163 
‘ in declination could not be followed with sufficient accuracy to 
secure the best results. Figure 7 (pl. 5) shows two views of the 
region around Aristarchus (indicated by an arrow), one made with 
yellow, the other with ultra-violet light. The dark deposit to the 
right of the bright crater comes out very clearly in the latter. The 
markings to the right of this region are quite different in the two 
pictures. Immediately below the pictures of the moon are three 
photographs made of two samples of volcanic “ tuff’? arranged one 
upon the other, with the crater Aristarchus marked with white chalk 
(as a check upon the exposure). The left-hand picture was made 
with yellow light, and the central specimen is lighter than the one 
surrounding it. The right-hand one was made with ultra-violet, 
and shows the central specimen distinctly darker. The middle 
picture was taken with violet light, which shows the two specimens 
of very nearly the same luminosity. I made an analysis of the 
fragment of tuff which photographed dark in ultra-violet light, and 
found that it contained iron and traces of sulphur. Photographs of 
rocks stained with iron oxide did not show the required peculiarity, 
and I accordingly attributed the result to the sulphur. A light deposit 
of sulphur was formed on the surface of a piece of light-gray rock by 
directing a fine jet of sulphur vapor against it. The deposit was so 
slight that absolutely no trace of it could be detected by the eye. The 
specimen was then photographed with yellow, violet, and ultra- 
violet light, and it was found that the deposit was quite invisible in 
the first picture, faintly visible in the second, and quite black in the 
third—precisely the peculiarity shown by the deposit surrounding 
the crater Aristarchus. Plate 6, a, b, c, show the gradual appearance 
of the deposit, which is an oval spot in the center of the specimen. I, 
feel inclined, therefore, to attribute this spot to an extensive deposit 
of sulphur, resulting from vapor ejected from the crater. The shape 
and vast extent of the deposit has always suggested to me that it 
resulted from material driven out in a volcanic blast.] 
Returning now from the moon to the physical laboratory, we will 
consider a further phenomenon which has been discovered and studied 
by means of photography in the ultra-violet region. The vapor of 
mercury has an absorption band in this region at wave length 2536, 
which I have made the subject of a somewhat extended investigation. 
At low pressures the line is very narrow, resembling one of the D 
lines of sodium, and I have detected its presence in mercury vapor 
at room temperature by employing a tube 3 meters long closed with 
quartz plates. It occurred to me that this vapor might prove to 
be the substance which I have long sought for the study of what I 
have named resonance radiation, i.e., a re-emission of light by absorp- 
ing molecules, of precisely the same wave length as that of the light 
absorbed. Sodium vapor was found to exhibit the phenomenon, 
