LANTERN SLIDES. 
153 
is to be kept down, and the tone obtained will be cold, 
a good blue-black. 
A. Iron protosulphate ... ... ... 140 grains. 
B. 
Sulphuric acid 
Water 
Potass, citrate 
Potass, oxalate 
Hot water 
1 minim. 
1 ounce. 
136 grains. 
44 >> 
1 ounce. 
The developer consists of 1 part of A. to 3 parts of B. 
With chloride plates the image usually comes up 
rapidly, and gains density slowly; time must be allowed 
for the acquisition of density, as much of it is apt to 
disappear on fixing. The plates are fixed in hypo¬ 
sulphite of soda, one part to five of water. It is a good 
plan to develop a plate to considerable density, and, if 
necessary after fixation, to reduce the density with the 
ferricyanide reducing agent given on page 128, or 
with the following, due to Mr. Cowan : 
Solution of iron perchloride, B.P. ... v ounce. 
Hydrochloric acid ... .. ... 1 ,, 
Water to ... ... ... ... 20 ounces. 
Soak for a short time in this, wash, and then immerse 
in fresh hypo solution. If any scum is found on these 
plates after any of these processes, it may be at once 
removed by the acid alum solution (page 124). 
Gelatino-bromide Slide Plates. 
These are the plates generally used for slide-making, 
the emulsion on them being practically of the same 
nature as that on bromide paper. The less sensitive 
the plates are for our purpose the more likely are we to 
succeed with them. We do not hesitate to recommend 
for trial the “ black tone ” plates of Messrs. Cadet.t and 
