EXPEKIMENTS WITH INVISIBLE LIGHT WOOD. 163 



in declination could not be followed with sufficient accuracy to 

 secure the best results. Figure 7 (pi. 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 

 pietures. 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 G, 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.] 



Keturning 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, 



