If the subject be a sitter, it is of the utmost importance to proceed as quickly as possible, as the identical position must be retained movelessly till both pictures are completed. This (in my experience) is scarcely practicable with collodion pictures, unless by the aid of an assistant and two levelled developing-stands in the dark closet; for the time occupied by starting the first picture on its development, and preparing the second glass plate (scarcely less than three or four minutes), will be a heavy tax on the quiescent powers of the sitter. This difficulty is avoided by adopting the Daguerreotype process, as the plates can be prepared beforehand, and need not be developed before both pictures are taken. In this case the only delay between the pictures is in the shifting the position of the camera. This is readily done by providing a table of suitable height (instead of the ordinary tripod), on which an arc of a circle is painted, having for its centre the place of the sitter. If the sitter be at the distance of eleven or twelve feet (my usual distance with a 3¼ inch Voightlander), the camera need not be moved more than ten or twelve inches; and even this distance produces some visible distortion to an accurate observer. 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. COKELY. _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. WILLIAM CROOKES. 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.