Fig. 1.
[F] The above description not perhaps being perfectly clear, and having been unable to obtain any further description, I give a sketch (Fig. 1.) of an apparatus found to answer well for the filtration of gelatine solutions and other viscid bodies. A is a glass flask, B a rubber stopper, C a funnel containing a filtering medium of washleather, D a tube attached to a three-way cock E; one hand manipulates the latter, while the other works the piston F of a syringe G.—Trans.
Printing Frames of different sizes will be found to hand in the photographic studio, and may be utilised without alteration for printing the collotype plates, if they are deep and strong enough to bear the necessary pressure, which is usually applied through the medium of springs; these are better removed, and wooden wedges inserted in their stead between the cross-bars and the loose wooden back of the frames, as by these means far more pressure may be applied. By lifting the one half of the hinged back of the printing frame an examination by transmitted light of the collotype plate may be made and an experienced operator will in this manner judge the exposure of the plate.
The Actinometer is, however, recommended, particularly for a beginner, as it greatly aids in forming a correct idea of the exposure.
The Drying Box is of great importance to the successful working of the process. The opinions of the various practitioners with regard to the temperature at which the drying of the plates should be effected differ as widely as upon the advisability or otherwise of admitting a current of air through the box during the operation. The drying should be completed as rapidly as possible from the commencement of the operation, care being taken that the heat never exceeds 50°C. Many plate-makers simply dry the plates in an open apartment—of course, only illuminated by a non-actinic light—simply placing the plates on a horizontal surface, which may be maintained at the temperature indicated by a water bath, a lithographic stone, or merely a cast-iron plate arranged in a suitable manner for heating from below. This method of drying is open to many objections: the surface of the plate is seldom free from dust, and the gelatine coating is too liable to irregularities from draughts admitted to the apartment during the process. They are more frequently dried in specially-constructed boxes provided with screws for accurately levelling the plates, and through which only a small circulation of air takes place. These boxes are usually rectangular in shape, the upright sides being of wood and the bottom of sheet iron. The lid is an open framework covered with a close orange or black cotton material, the whole standing upon four iron legs over a spirit or gas flame. In the upper part of the box a thermometer is fixed, about the centre of either the side or lid, in such a position that it may be readily observed without the necessity of opening the box. Strong horizontal iron bars are placed across at about the centre, and are provided with thumbscrews, upon which may be placed a plate of glass bearing a circular spirit level, by which means the plates may be levelled with the greatest accuracy. The sheet-iron bottom of the box being heated unevenly, it becomes necessary to mitigate this inconvenience as far as possible, which is easily done by covering the plate to a depth of about half-an-inch with dry river sand, over which should be placed tissue paper to keep down any possible dust.
Fig. 2.
An Automatic Regulator of practical value is that devised by Ruegheimer. It consists of a glass tube, A, A1, A2. A1 is closed with an indiarubber stopper, through which passes a glass tube B, the lower end of which is cut off at an angle. It is attached to the gas supply pipe. The tube C is connected to the burners. To D is attached, by means of rubber tubing, a glass bulb F, which is placed inside the drying-box. G G is mercury, and H a rubber stopper by which the pressure on the mercury and quantity of air in F may be regulated. The action of the instrument is obvious. The gas passes down B, over the surface of the mercury and by the tube C to the burner. On the bulb F reaching a certain temperature, the mercury will allow just sufficient gas to pass from the tube B to maintain the box at a given heat. If it should fall, the mercury recedes from the aperture of the tube B, a larger quantity of gas passes to the burner, and the temperature is restored to a normal degree. If the air in the ball F expands to too great an extent, the mercury rises, and would eventually entirely close the aperture and cut off the gas supply, unless the tube B is provided with a small hole acting as a by-pass. The tube B may be moved up and down through the rubber stopper at A1 as a means of adjustment.
