correspondence. I obtain all the information I can from every source, then
try, and judge for myself. At worst, you only spoil a few sheets of paper,
and gain experience.

I perfectly agree with DR. DIAMOND, that it is much better not to wash the
collodion pictures after developing; but pour on about one drachm of sat.
sol. hypo. at once, and then, when clear, plenty of water; and let water
rest on the surface for an hour or more, before setting on edge to dry.

HENRY WILKINSON.

_Collodion Negative._--Can you inform me how a collodion negative may be
made? that is, how you can ensure the negative being always of a _dense
enough character to print from_. This is rarely the case.

F. M.

_Developing Collodion Process._--I use to develope my collodion pictures M.
Martin's plan, _i. e._ a solution of common copperas made a little acid
with sulphuric acid. This answers very well and gives to the pictures,
after they have been exposed an hour or two to the atmosphere, a
silver-like appearance: but this copperas solution seems to destroy the
_glass_ for using _a second time_, inasmuch as a haziness is cast upon the
glass, and its former enamel seems lost, not to be regained even by using
acids. The hyposulphite also seems to be affected by this manner of
developing the {605} pictures after a short time, which is not the case
with pyrogallic acid. The hypo., when thus affected with the copperas,
appears also to throw a mist over the picture, which new hypo. does _not_.
I should esteem it a favour if any of your numerous readers could inform me
the cause of this.

A. A. P.

_An iodizing Difficulty._--May I request the favour, from some one of your
numerous photographic correspondents, of a solution to the following
apparent enigma, through the medium of "N. & Q."?

Being located in a neighbourhood where there is a scarcity of water in the
summer months, I lately took advantage of a pool in a running stream, which
ran at the bottom of the grounds of a friend, to soak my calotype papers
in, subsequent to having brushed them over with the solution of iodide of
silver, according to the process recommended by SIR W. NEWTON. One-half of
the batch was removed in about two hours and a half, being beautifully
clean, and of a nice light primrose colour; and in consequence of an
unexpected call and detention longer than I had anticipated, the other half
was left floating from two o'clock P.M. until seven or eight in the evening
(