368 



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 T43 4 fluid ounces. 



Commercial nitric acid 1'37 4 



Sulphuric acid 1-82 8 



When the temperature of the mixture had cooled down to 50 C., 

 one sheet of Swedish filtering-paper, torn up into small pieces, was 

 completely immersed in the mixture, and allowed to remain therein for 

 about half an hour. It was then thrown into a large pail of water, 

 and the paper removed and placed on a sieve under a running tap 

 for a quarter of an hour ; after washing in very dilute solution of 



