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Three-Color Transparencies and Lantern Slides 447
gelatin with hydrofluoric acid, then the glass scrubbed and polished with a paste of tripoli 1 part, whitening 10, and 2 per cent caustic soda solution, and then dried. Hot water would be quite as efficient as the acid for cleaning the glass.
The plates should be coated on a whirler at about 400 revolutions per minute. The scoured plate should be attached to the whirler, rinsed under the tap and given a whirl to free it from water, and a small pool of the sensitive mixture poured on to the middle of the plate, guided to the edges and the plate drained and a second lot of fish glue poured on and then the plate whirled. It should then be removed from the whirler, the back cleaned and dried before a hot fire or over a gas burner, but in the latter case the film must be held away from the flame or the fumes of the burnt gas would fog the plate. If rapidly dried at about 27° C, the plate was about twice as sensitive as when dried at half that temperature. The plate should be warmed before exposure, which varied from about 1 minute in the middle of the day in June to 6 minutes in December in full sunlight. An arc could be used and the exposure to an enclosed arc, 200 volts, 4.5 amperes, alternating, was 12 minutes at 12 inches distance.
Development was effected by soaking in cold water for about 30 seconds, then rinsing under a tap or strong spray. A fairly complete list of dyes and the colors obtainable therewith are given, but as many are of no interest to the color worker, the following are selected for trial: for deep blue, soluble blue 1 and methyl violet 3, then malachite green 5 ; for blue, soluble blue 5 and oxalic acid 1 ; for peacock blue, soluble blue 5 and naphthol green 5, then malachite green 5 ; for blue-green, naphthol green 1, then malachite green 5; for lemon yellow, aurantia 2 and auramin 0.3; for orange-red, rose Bengal 10, then auramin 0.3; for scarlet, rose Bengal 10 then auramin 0.3, then rose Bengal 10; for deep red, dye scarlet as in the last case and then with chrysoidin 2 ; for pink, rose Bengal 10. The numbers after the names of the dyes refer to the strength of the solutions in percentages. The solutions should be applied in succession and this is indicated by the word "then."
For obtaining superposed images in colors Bawtree preferred to use ordinary cyclist's rubber solution thinned down with benzol or coal tar naphtha to the consistency of cream, and poured a small pool on the plate at one corner and after coaxing it over the plate, whirled at about 120 revolutions per minute in front of a hot fire, and allowed to remain there for about an hour. Then he applied a coat of enamel collodion, and when this was dry, again coated with fish glue and treated for the subsequent impressions.
C. K. Teamer and E. E. Miller20 proposed to use Kodak film, and presumably sheet and not roll film, as nothing was said about the back coating of gelatin. This may be manipulated in daylight, as the silver