Mr. W. Crookes on the Photography of the Moon. 229 
telescope. This was done by means of slow-motion screws attached 
to the right ascension and declination circles, which are each 4 feet 
in diameter. The finder had an eyepiece with a power of 200 applied 
to it, having cross wires in its field. 
The modus operandi of taking the picture was as follows :—The 
telescope having been moved until the moon’s image was in the centre 
of the focusing glass, the water-mill was turned on and the dark 
slide containing the sensitive collodion plate was substituted for the 
ground glass. Mr. Hartnup then took his station at the finder, and 
with a tangent rod in each hand, by a steady and continuous move- 
ment, kept the point of intersection of the cross wires stationary on 
one spot on the moon’s surface. 
When the motion was most perfectly neutralized, I uncovered the 
sensitive plate at a given signal and exposed it, counting the seconds 
by means of a loud-ticking chronometer by my side. 
From the ease with which on my first attempt I could keep the 
cross wires of the finder fixed on one point of the moon by means 
of the tangent rods, I confidently believe that with the well-tutored 
hands and consummate skill which guided this noble instrument, the 
moon’s image was as motionless on the collodion plate as it could 
have been were it a terrestrial object. 
The negatives which I obtained by these means were exquisitely 
beautiful, and so minute that I could not obtain paper with a suffi- 
ciently fine surface whereon to print copies which would do them 
justice. It was evident that they would bear magnifying several 
diameters and still remain sharply defined. The expense of carrying 
out this design here stopped me, when by the kind advice of Pro- 
fessor Wheatstone I applied to the Royal Society, whose munificence 
has so frequently been the cause of bringing to a successful termina- 
tion investigations of the highest importance. 
A half-plate photographic combination of lenses, by Ross, was 
screwed the reverse way into a large sliding camera body 10 inches 
high by 11 inches wide, and capable of sliding from 18 inches to 
3 feet long. At the end of the sliding body opposite to the lens, was 
a groove to admit either a focusing glass or a dark slide for the 
sensitive plate. A smaller camera body was screwed into the other 
end of the brasswork of the lens, having also a groove in front to 
admit of a sliding box capable of holding the small negatives. A re- 
flector was placed in front of all, so arranged as to move in altitude 
round a centre, and, being fixed in any required position, to reflect 
the diffused light of the sky through the negative and lens parallel 
with the axis of the Jatter. 
Preliminary trials showed me that there was no good gained by 
magnifying the small pictures. more than about 20 times, as after 
that the individual parts begin to get confused and indistinct; this 
magnifying cannot, however, be effected at once. In the small nega- 
tives the lights and shades are the reverse of what they are in nature, 
consequently a print on paper therefrom gives the light and shade 
correct. A photographic copy of a negative, however, produces a 
positive by transmitted light, and a print from this would have the 
