162 ANNUAL. REPORT SMITHSONIAN INSTITUTION, 1911. 



correction discernable in visible light; but when the photograph was 

 made with the ultra-violet, the erasure, otherwise invisible, showed as 

 a black smudge. The ultra-violet camera is evidently very much 

 more sensitive than the eye to the presence of traces of Chinese white 

 on the printed page, for so far as I could see every particle of the 

 pigment had been removed. Whether this has any bearing upon the 

 detection of forgeries has yet to be discovered. 



Another class of work in winch this comparative study is likely to 

 be of 'service is the photography of celestial bodies. For the full 

 moon the exposure through the silver screen was two minutes with 

 ultra-violet light belonging to the region 3000 to 3200. This length 

 of exposure necessitated an equatorial telescope with some means of 

 driving it to compensate for the moon's movement. The support for 

 my telescope was the framework of an old bicycle minus the wheels. 

 This carried a 4-inch refractor and a quartz-silver telescope, and by 

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

 accurately for half an hour. It will be seen at once (pi. 5) that there 

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

 the one which is shown us by the ultra-violet radiation. Nevertheless 

 in the neighborhood 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 tins crater 

 appears, and it is evident that there is in its neighborhood 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 centi- 

 meters square and 1 millimeter thick, heavily silvered. The plate 

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

 for the rays transmitted by 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 



