586 
pyrogallic acid will hardly bear any 
enlargement, though one made with 
sulphate of iron and a well-regu- 
lated exposure may be increased in 
diameter twenty-five times, without 
showing the granulations offensively. 
The influence of the structure of the 
collodion-film itself, too, is notice- 
able in pictures taken by the wet 
process; in the first place, being 
somewhat transparent, 1t permits a 
certain amount of lateral diffusion 
in the film, and a tendency to soften 
down the more minute details ; and, 
in the second place, while wet it 
has quite a perceptible thickness, 
which is much diminished in dry- 
ing, and the relation of the silver 
particles to one another changed. 
I have attempted to avoid the 
faults connected with structure of 
the film by substituting dry col- 
lodion, and more particularly tannin 
plates for the wet. But though 
during the exposure to the celestial 
object the sensitive plate presents 
a glassy surface of extreme thin- 
ness, yet an indispensable pre- 
liminary to evoking the latent 
image is to soak the plate in water, 
and this introduces the more in- 
jurious of the two objections above 
urged, It was while trying this 
process that I ascertained the ad- 
vantages that arise from warming 
the film during development,—the 
“hot-water process,” as it is called. 
The attempt was also made to da- 
guerreotype the original pictures at 
the focus of the telescope on silver 
plating, and also on si/vered gluss. 
In this case all lateral diffusion is 
entirely prevented, the light acting 
on a mathematical surface, and the 
relations of the film of silver to the 
glass not being disturbed by the 
-subsequent manceuvres. But prac- 
tically no advantage has arisen from 
these trials, because, as in the for- 
mer instance, the whites in the re- 
sulting picture are not formed by a 
continuous stratum of mercurial 
amalgam. That this is the case is 
proved by the fact that such da- 
guerreotypes can be copied by the 
electrotype, or a coating of isinglass, 
as was shown by Dr. Draper (‘ Phil. 
Notes and Correspondence. 
[ April, 
Mag.,’ May, 1843). This is the first 
occasion on which silvered glass has 
been used for photographic pur- 
poses, and it may be well to point 
out its advantages. Owing pro- 
bably to the perfect purity of the 
silver, it takes the coatings of iodine 
and bromine with uniformity all 
over; in silver plated by fire on 
copper, there used to be a tendency 
to insensitive spots, from the copper 
alloy coming out on the face of the 
silver, and so great was the annoy- 
ance, that, when my father was en- 
gaged in the experiments that led 
him to take the first portrait ever 
obtained from life, he was compelled 
to use sheets of pure silver alone. 
The light also seems to be able to 
impress the iodo-bromide in less 
time, and pictures of a rosy warmth 
are generally obtained. ‘The only 
precaution necessary in practising 
this method of daguerreotyping is 
to fix the plate—that is, dissolve off 
the excess of sensitive material— 
with an alcoholic solution of cyanide 
of potassium, instead of an aqueous 
solution of hyposulphite of soda. 
The latter tends to split up the film 
of silver from the glass here and 
there, while the former does not. 
The subsequent washing, too, is 
most safely conducted with diluted 
commen alcohol. The time of ex- 
posure is not, however, as short as 
in’ the wet-collodion process, at 
least six times the exposure being 
demanded ; while if less is given, 
and the development over mer- 
curial vapour be urged beyond the 
usual point, minute globules of mer- 
cury stud the silver all over, and 
ruin the proof, 
The faults arising from atmo- 
spheric disturbances are easily un- 
derstood. If an image of the planet 
Jupiter produced by a large tele- 
scope be allowed to move across a 
sensitive plate, and the plate be 
then developed, a dark streak nearly 
of the width of the image will ap- 
pear. If this streak is closely exa- 
mined, it will be observed that the 
passage of the planet seems to have 
taken place in an irregular way—by 
fits and starts as it were, and that 
