1911] on Recent Experiments icith Invisible Lie/ hi. is? 



operation of a little screw it was possible to follow the moon uccii- 

 ratelj for half an hour. It will be seen at once (Fiii'. 7) that there is 

 very little difference between the ordinary image of the moon and the 

 cue which is shown us by the ultra-violet radiation. Nevertlieless, in 

 the neighbourhood of Aristarchus, which is the brightest crater on 

 the lunar surface, the photograph taken with the ultra-violet rays 

 shows a dark patch which is absent on the one taken with visible 

 light. I made an enlargement of the region in which this crater 

 appears, and it is evident that there is in its neighbourhood a large 

 deposit of some material which can only be brought out by means of 

 the ultra-violet. These photographs of the moon make it appear 

 extremely probable that by carrying on experiments of this nature on 

 a larger scale we might get a good deal of new information as to the 

 materials of which the moon is composed. It is possible to examine 

 the igneous rocks of the earth under the different radiations, and then 

 compare them with the pictures of celestial objects obtained at the 

 same wave-lengths. I have found that some rocks, which when 

 illuminated by ultra-violet rays appear darker than others, are lighter 

 than the others in visible light. 



Note added October 1911. 



[I have had constructed a 16-inch mirror of 26-feet focus 

 which I have coated with nickel, for extending the study of the 

 ultra-violet photography of the moon and planets. This is now 

 being used in combination with a plate of the new ultra-violet 

 glass, 12 cm. square and 1 mm. thick, heavily silvered. The plate 

 was made by Zeiss, and I find that it is quite as transparent as quartz 

 for the rays transmitted l)y the silver filter. This reflector was 

 mounted on the 23-inch equatorial of Princeton University, and 

 some very fair pictures have been obtained, though the moon's 

 motion in declination could not be followed with sufficient accuracy 

 to secure the best results. Fig. 7 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 



