- Mr. W. Crookes on the Photography of the Moon. 231 
riod which, even were it prolonged four or five times, would not then 
be too severe a tax upon a steady and skilful hand and eye. 
Description of the Photographic Process. 
The glass employed for taking the original negative of the moon, 
was that known as “extra white colour patent plate,” that for the 
intermediate positives and large negatives was ordinary patent plate. 
Cleaning the surface, which is an operation of especial importance, 
was effected in the following manner. 
The glasses were dipped into and then well rubbed over with a hot 
solution of caustic potassa ; then, after washing with water, they were 
transferred to hot nitric acid (one part strong acid to three of water), 
where they were allowed to remain for about half an hour. 
A piece of soft wash-leather was plentifully rinsed, first in a warm 
dilute solution of carbonate of soda, afterwards in clean water, and 
then well wrung until all the superfluous water was squeezed out, 
The glass plates were taken from the nitric acid and rinsed in abun- 
dance of clean water, and then rubbed well on every part with the 
damp leather. This removed most of the superficial moisture ; and the 
final drying was effected by means of another piece of wash-leather, 
_ prepared the same as before, but allowed to become perfectly dry. 
Just previous to using, the plates, held in a pneumatic plate-holder, 
had the last polish given to them by briskly rubbing with a warm 
piece of fine diaper (which had also been previously washed in soda 
and water, and then well rinsed and dried) until the moisture con- 
densed from the breath evaporated evenly and uniformly, especially 
guarding against the slightest contact between the surface of the 
glass and the fingers. 
The plate was now held with its clean side downward until the 
collodion was about to be poured on, and every particle of dust 
(which was easily seen by bringing the source of light, the under 
surface of the plate and the eye, nearly in the same line) was gently 
wiped off by passing a warm piece of fine cambric lightly across. 
Care was also taken to have the atmosphere of the room as free 
as possible from floating particles, and the dried collodion usually 
adhering to the neck of the bottle was scrupulously removed. 
The collodion was poured on and the plate rendered sensitive in 
the usual way. As the temperature both of the equatorial and ope- 
rating rooms was seldom far from the freezing-point, the great dimi- 
nution of sensitiveness, which that circumstance would have occa- 
sioned, was obviated by having the nitrate of silver bath and deve- 
loping solution warmed to about 30° C., and also by slightly warm- 
ing the plates before using. The source of light was a fishtail gas 
burner in the outer room, and shining close to the orange glass 
window of the dark room. 
The soluble paper for the collodion was prepared in the following 
manner :—a mixture was made of 
sp. gr. 
Commercial nitrous acid ...... 1°43 4 fluid ounces. 
Commercial nitric acid........ 1°37 4 e 
BONPRUTIC BGI sae n 05 sine vis we os 1°82 8 ” 
