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 which 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 (pl. 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 this 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. Ihave 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 
