Cros and Vergeraud have worked out a process for obtaining images so as to have a positive impression from a positive plate, and a negative print from a negative original. The process is based on the following circumstances: The easy reduction of soluble bichromates mixed with certain organic substances, and the relative insolubility of silver bichromate. Suitable paper is covered with a solution of 2 dr. ammonia bichromate, and 15 dr. grape sugar, dissolved in 100 dr. water; when dry, it is exposed to light under a positive. As goon as the yellow paper becomes grey, it is removed, and immersed in a 1 per cent. silver bath, to which 10 per cent. acetic acid has been added. The image will immediately appear of a ruddy hue, due to the silver bichromate. The print, on being washed, retains the red impression of the insoluble bichromate, which becomes dark brown on exposure to sunlight. On submitting the print when dry to the fumes of sulphuretted hydrogen, or dipping in a solution of copper sulphite and potash, it becomes black.
The latter process is preferable. (Photo. News.)