Hammersmith.
P.S.--I see that in the tables R. E. has given, he has nearly doubled the
strength of my iodine bath. It should be twenty-four grains to the ounce,
instead of forty-four; and he has entirely left out the iodine.
_India Rubber a Substitute for Yellow Glass._--I think that I have made a
discovery which may be useful to photographers. It is known that some kinds
of yellow glass effectually obstruct the passage of the chemical rays, and
that other kinds do not, according to the manner in which the glass is
prepared.
I have never heard or read of India rubber being used for this purpose; but
I believe it will be found perfectly efficient, and will therefore state
how I arrived at this conclusion.
Having occasion to remove a slate from the side of my roof, to make an
opening for my camera, I thought of a sheet of India rubber to supply the
place of the slate, and thus obtain a flexible waterproof covering to
exclude the wet, and to open and shut at pleasure. This succeeded
admirably, but I found that I had also obtained a deep rich yellow window,
which perfectly lighted a large closet, previously quite dark, and in which
for the last ten days I have excited and developed the most sensitive
iodized collodion on glass. I therefore simply announce the fact, as it may
be of some importance, if verified by others and by further experiment. I
have not yet tested it with a lens and the solution of sulphite of quinine,
as I wished the sun to shine on the sheet of India rubber at the time,
which would decide the question. However, sheet India rubber can be
obtained of any size and thickness required: mine is about one-sixteenth of
an inch thick, and one foot square; and the advantages over glass would be
great in some cases, especially for a dark tent in the open air, as any
amount of light might be obtained by stitching a sheet of India rubber into
the side, which would fold up without injury. It is possible that gutta
percha windows would answer the same purpose.