324 Application of Photography to the Self-registration 
and the glass rod again passed lightly over it, which diffuses the 
solution over it; after which it is applied to the cylinder as be- 
fore; by this process the sensibility of the paper is said to be much 
increased, and it keeps acleaner surface. ‘To present at one view 
the different chemical processes, we will suppose the cylinder to 
be now left to complete one revolution (of twelve hours) or two, 
as the case may be, the paper has then to be removed, the impres- 
sion to be developed, and then fixed. For this purpose a common 
large dish is required, which should be placed before a fire until 
moderately warm. Excluding daylight as before, remove the 
cylinder carefully from its supports, take off the paper with as 
little fingering as possible, and lay it on the dish; no indication 
of the trace will be perceived, unless the disk be more than duly 
heated, in this case the lines sometimes appear faintly without 
farther treatment, but the paper darkens too much afterwards. It 
has to be now brushed with 
(3.) Twenty grains of crystallized gallic acid, dissolved in 
one ounce of distilled water* when it appears in a minute or two. 
When the impression is sufficiently distinct, which will generally 
be in the space of five minutes, the gallic acid must be washed 
off by repeated sluicings with soft water, assisted by a soft brush. 
In a cold place, or with a cold dish, a longer time is required ; in 
short the whole process seems to succeed best, like most photo- 
graphic processes, at a high temperature, although the trace has 
been developed at a temperature low enough to convert the solu- 
tion into a film of ice. When the uncombined nitrate of silver 
and gallic acid are thoroughly washed off, the light may be ad- 
mitted without danger ; it then remains to fix the impression by 
transferring the paper toa clean dish or board, and brushing it 
with about one table spoonful of the usual solution, namely, . 
(4) Twelve grains of hyposulphite of soda to one ounce of 
distilled water. It will be observed that rather a larger quantity 
of this solution is laid on the paper than of the previous ones. 
If an insufficient quantity be applied, or it be not uniformly dis- 
tributed, the paper is apt to acquire a dirty brown stain, in patches, 
passing into black, which latter it ultimately becomes when no 
hyposulphite is applied ; in this case, the margin of the previously 
dark lines becomes the lightest portion ; after allowing the hypo- 
sulphite a few minutes to act, it must be thoroughly washed off, 
by repeated sluicings of water, assisted as before, with a soft brush, 
otherwise it enters into a new combination which spoils the spe- 
cimen: the paper may then be placed between the folds of clean 
white blotting paper until dry, when the process is completed. 
We may now return to the mechanical details. The mag- 
net employed is about two feet long, one inch and a quarter 
Pe sieNe apy a eth cael thialilinaa allie PT er 
_* One ounce of water will not hold twenty grains of gallic acid in solution at or- 
dinary temperatures; it: is necessary to dip the bottle in boiling water, or heat @ 
small of surcharged solution, in a test tube. 
