Volltext Seite (XML)
May 15, 1885.] THE PHOTOGRAPHIC NEWS. 307 graph me in my hat. Not knowing what the man wanted it for, I consented; but judge of my astonishment when I saw it not an hour ago pinned on to a hat in a milliner’s window in Regent Street, and the hat labelled with my name! And not only that, but I saw the photographs of you all exhibited in the same way. 4th Ditto. Really, I don’t see what there is to make a fuss about. It’s a very good advertisement. So far as I am concerned, I don’t care one pin. 5th Actress. No, because yours is the newest style ; but (very bitterhf) mine is one of last year’s shape, and the photograph is pinned on to a dowdy thing in the back row ! [A light breaks in suddenly. It was clear that the perfidious Smiley had somehow a contract with the wholesale milliners to secure portraits of actresses to act as advertisements—not of the fair sitters themselves, but of their hats! Smiley, after all, was not so mad as he seemed.] GELATINE EMULSIONS, AND HOW TO MAKE THEM. BY W. E. DEBENHAM. NO. II. IN making emulsion on Dr. Maddox’s plan, there was, with the proportions of silver and gelatine originally given, so much crystallization of the nitrate of potash retained in the film, that the process as it stood has been regarded by many as impracticable ; and as it had been the prac tice in collodion emulsion to remove the by-product salts by washing, similar means was adopted with gelatine emulsion. The method of removing the soluble salts from gelatino- bromide of silver that was first generally employed, was that of dialysis. This process depends upon the fact that solutions of crystalloid bodies, such as the nitrates produced by the double decomposition of silver nitrate, and the bromide of the metal or base employed in making to form bromide of silver, will pass through a membranous tissue ; whilst colloid bodies in solution, such as gelatine, do not pass through. To dialyse an emulsion, a “ septum ” or piece of parchment, or parchmentised paper, is strained over a hoop or ring of glass, and into the tray thus formed emulsion is poured to the depth of about a quarter of an inch. The tray is suspended in a vessel containing warm water, which water stands at about the same height out side the dialyser that the emulsion stands within it. Some prefer to keep the level of the emulsion inside higher than that of the water outside. In the course of a few hours the greater part of the soluble salts will have passed through the septum, and the emulsion will, in consequence of the prolonged digestion, have gained in sensitiveness. At one time, when the effect of digestion in increasing sensitiveness was not duly appreciated, the accelerative action was attributed to the removal of the soluble salts, and particularly of the excess of bromide. The plan of removing soluble salts from emulsion by dialysis is obvi ously unsuited for any but small quantities, and has beer, superseded by other methods. The fact that gelatine emulsion is solid or liquid accord ing to temperature, suggested the idea that the soluble matters might be extracted by exposing flakes or sheets of the set emulsion to the action of water. Since the action takes place from the surface, it is necessary that no part of the emulsion shall be far from the surface ; that is to say, that the pieces must be small, or, if in sheets, that the sheets shall be thin. A plan at one time recommended was to place a small quantity of emulsion in a large bottle, which was to be turned about until the gelatinous mix ture had “set” upon the sides of the bottle; this was then filled with cold water, and fresh water was kept running in to the bottom of the bottle, led there by a pipe of india-rubber, or the water was changed at intervals. This method sometimes called for the exercise of consider able patience during the “ setting,” and was only suitable for small quantities. The plan of washing emulsions in general use is that which was published in 1878 by Messrs. Wratten and Wainwright. The essential feature of this method is the dividing of the set emulsion into fine shreds so as to present a large surface to the water. To accomplish this, the emulsion is removed from the dish or vesssel in which it has set, to a bag, or simply to the middle of a square of mosquito netting or canvas, such as is used for Berlin wool work. The mouth of the bag is then closed with one hand, or the corners and edges of the square brought together like a pudding bag. The sack of emulsion is now held under water—this is absolutely essential—and squeezed with the other hand until the emulsion has all passed through the interstices of the fabric. This must be done in a clean eathernware basin, and the water and shreds of emulsion are next poured into a hair-sieve. The water runs through, leaving the shreds of emulsion on the sieve. The sieve is now to be placed in a basin in which it will stand at such a height that the water in the sieve shall be at least twice or three times the depth that the emulsion alone would occupy. In placing the sieve in the basin, let Fig. 1. the latter be first filled with water, and the sieve be rapidly forced down to its position. This will cause the air con tained under the hair cloth to find its way through the Fig. 2. interstices ; without this precaution the sieve may be upset, and some of the emulsion spilt by the escape of the air at one side. A very convenient appliance for those who have no water supply in the dark-room, has been exhibited by Mr. A. L. Henderson, and is manufactured at the Lambeth Potteries—perhaps only to order—and shown in fig. 2. The sieve here rests upon a ledge formed in an earthenware vessel, which has an opening near the bottom for carrying off the washing water, and is fitted with a lid, the side of which drops into a channel formed for the purpose, so as to prevent light from entering the interior.