73 
removed the eyepiece of the telescope, and in its place a 
board was fixed having a screw adjustment, so that a plate- 
holder could be moved backwards and forwards on the board 
(graduated to tenths of an inch) for the purpose of finding 
the actinic focus, which was three quarters of an inch beyond 
the visual. He found that this position of the chemical focus 
was variable, owing, as he thought, to the varying distance 
of the moon from the earth, but, as suggested by Mr. De la 
Rue, it might arise from the length of the telescope tube 
having altered through change of temperature. 
In 1858 Mr. De la Rue read an important paper before the 
Royal Astronomical Society, from which it appears that the 
light of the moon is from two to three times brighter than 
Jupiter, while its actinic power is only as six to five, or six 
to four. On December 7th, 1857, Jupiter was photographed 
in five seconds and Saturn in one minute, and on another 
occasion the moon and Saturn were photographed just after 
an occultation of the planet in fifteen seconds. 
The report of the council of the Royal Astronomical Society 
for 1858 contains the following remarks : — 
“ A very curious result, since to some extent confirmed by 
Professor Secchi, has been pointed out by Mr. De la Rue, 
namely, that those portions of the moon’s surface which are 
illuminated by a very oblique ray from the sun possesses so 
little photogenic power, that, although to the eye they appear 
as bright as other portions of the moon illuminated by a more 
direct ray, the latter will produce the effect called by photo- 
graphers, solarisation, before the former (the obliquely-illu- 
minated portions) can produce the faintest image.” 
And the report also suggests that the moon may have a 
comparatively dense atmosphere, and that there may be vege- 
tation on those parts called seas. 
At the meeting of the British Association at Aberdeen, in 
1859, Mr. De la Rue read a very valuable paper on celestial 
photography. An abstract of this paper was published at the 
