So far we have only treated upon reverse negatives, either obtained at once or reversed afterward. It often happens, however, that we have an ordinary negative, which is required to be reversed. This negative may be a valuable one, and the risk involved in stripping it be too great.
Another simple method of obtaining a reversed negative is by means of the powder process. Although this process is an old one, it appears to be but little known, for what reason we have never been able to define. It is by no means difficult, and by its means a negative can be obtained direct from a negative without the intermediate positive transparency.
The principle of the process is this: An organic tacky substance is sensitized with potassium dichromate, and exposed under a reversed positive to the action of light. All those parts acted upon become hard, the stickiness disappearing according to the strength of the light action, while those parts protected by the darker parts of the positive retain their adhesiveness. If a colored powder be dusted over, it will be understood that it will adhere to the sticky parts only, forming a visible image, the same being a reproduction of the positive printed from. The process is very useful for the production of lantern slides and transparencies, or for the reproduction of negatives. Any of the following formulæ may be employed for the manufacture of the organic substance:—
Solution A.
| Gum arabic | 25 grammes |
| Grape sugar | 60 grammes |
| Purified honey | 15 grammes |
| Alcohol, 40 deg | 15 c.c. |
| Water | 60 c.c. |
Solution B.
| Saturated solution of ammonium dichromate. |
Two solutions to be mixed together before using in proportions 15 A, 25 B, 50 water.
Woodbury's Formula.
| Gum arabic | 60 grains |
| Glucose | 45 grains |
| Glycerine | 10 minims |
| Potassium dichromate | 30 grains |
| Distilled water | 2 ounces |