ing action on them;
and when the liquid becomes saturated, a few crystals of fresh hyposulphite
will renew its action.

F. MAXWELL LYTE.

Florian, Torquay.

P. S.--In answer to a COUNTRY PRACTITIONER, he will find great assistance
in choosing his lens by laying it on a sheet of blue wove post paper, when
he will immediately perceive the slightest yellow tinge in the glass, this
being the fault which frequently affects many well-ground and well-made
lenses. Of course, for sharpness of outline he must be guided entirely by
experiment in the camera; but where weakness of action exists, it most
frequently arises from this yellow colouration, and which the manufacturers
say is very difficult to avoid.

[MR. LYTE having sent with his communication a positive prepared in the
manner described, we are enabled to corroborate all he says as to the
richness and beauty of its tints.]

_Cheap Portable Tent._--M. F. M. inquires for a cheap and portable tent for
working collodion out of doors. I have been using one lately constructed on
the principle of Francis's camera stand. It has a good size table, made
like the rolling patent shutters; and it is not necessary to stoop, or sit
down at your work, which is a great consideration on a hot day: you may get
them of any respectable dealer in photographic apparatus; it is called
Francis's Collodion Tent.

H. D. FRANCIS.

_Rev. Mr. Sisson's New Developing Fluid_ (Vol. vii., p. 462.).--The REV.
MR. SISSON's developing fluid for collodion positives, the formula for
which was published in the last Number of "N. & Q.," is merely a weak
solution of the protonitrate and protosulphate of iron. It does not, as he
seems to think, contain any lead; for the whole of the latter is
precipitated as sulphate, which the acetic acid does not dissolve even to
the smallest extent: and MR. SISSON will find that an equivalent proportion
of the nitrate of baryta will answer equally as well as the nitrate of
lead.

I have myself for a long time been in the habit of using a weak solution of
the protonitrate of iron in conjunction with acetic acid for positive
pictures; for, although I do not consider it so good a developer as that
made according to the formula of DR. DIAMOND, it produces very good
pictures; occupies very little time in preparing, and will moreover keep
good for a much longer time than a more concentrated solution would.

J. LEACHMAN.

20. Compton Terrace, Islington.