228 Royal Society :— 
there are few good photographs of the moon yet in existence. It 
was my good fortune in the autumn of 1855 to obtain several ex- 
cellent’ pictures of this kind, and since these form the starting-point 
of the work which, by the assistance of a grant from the Donation 
Fund of the Royal Society, I have been pursuing during the greater 
part of the last year, a detailed account of the means employed for 
their production will not, I think, be considered out of place here. 
The telescope in which these pictures were taken is the magnifi- 
cent equatorial at the Liverpool Observatory. This, together with 
all the resources of the establishment, was placed at my disposal by 
my kind friend Mr. Hartnup, to whom it is but due to state, that, 
were it not for the invaluable assistance afforded me by his sterling 
advice as well as steady hand, the results would not have been worth 
keeping. 
The mounting of the equatorial is quite unique ; the polar axis and 
telescope together weigh about five tons, and whilst all parts are so 
truly and smoothly fitted that this enormous mass is moved equato- 
rially by means of a small water-mill with such marvellous accuracy, 
that a star viewed through it appears absolutely stationary, its firm- 
ness is such that a hard blow against the side merely produces a 
scarcely perceptible momentary deflection. The object-glass is 8 
inches in diameter, and has a sidereal focus of 12°5 feet—the dia- 
meter of the moon’s image in this focus being about 1°35 inch. 
The eyepiece was removed, and in its place the body of a small 
camera was attached, so that the moon’s image might fall upon the 
ground glass or sensitive film in the usual manner. Much labour 
had been saved me in finding the true actinic focus, by several pho- 
tographers of Liverpool, who were working for some time on the 
same subject when the British Association met in that city in 1854. 
They found that the object-glass had been over-corrected for the 
actinic rays—the plate being required to be placed at a distance of 
0:8 of an inch beyond the optical focus: a few experiments were 
sufficient to enable me to verify this result. 
During the time above referred to, and frequently since, Mr. Hart- 
nup had taken many hundreds of pictures with chemicals recom- 
mended by various persons, but had not succeeded in obtaining a good 
negative at all, and not even a positive with a less exposure than 
from half a minute to a minute. As I succeeded in taking dense 
negatives in about four seconds, with the temperature of the room 
below freezing and the moonvat a considerable distance from the 
meridian, and as I attribute the greater sensitiveness which I ob- 
tained to the great purity of the materials I employed, I think it 
right to give, after the mechanical arrangements are described, an 
account of the way in which these were prepared. 
The clockwork movement was only sufficient to follow the moon 
approximately when on the meridian, but as the pictures were nearly 
all taken when she was some distance past the meridian, and when 
consequently the declination and atmospheric refraction were changing 
rapidly, it was necessary, notwithstanding the short time required 
to take the pictures, to correct for the imperfect motion of the 
