The second levelling stand is required when using the collodion process, because the second { 71} picture will be ready for development before the developing and fixing of the first has set its stand at liberty.
_Mr. Crookes' Wax-paper Process_ (Vol. vi., p. 613.).--R. E. wishes to know the exact meaning of the sentence, "With the addition of as _much free iodine_ as will give it a sherry colour." After adding the iodide of potassium to the water, a small quantity of iodine (this can be proctored at any operative chemist's) is to be dissolved in the mixture until it be of the proper colour.
The paper is decidedly more sensitive if exposed wet, but it should not be washed; and I think it is advisable to have a double quantity of nitrate of silver in the exciting bath. I have not yet tried any other salt than iodide of potassium for the first bath; but I hope before the summer to lay before your readers a simpler, and I think superior wax-paper process, upon which I am at present experimenting.
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.