In the process of collotype printing on parchment, as employed in the photo autocopyist process, a sheet of parchment, coated with gelatine and sensitised with bichromate of potash, is exposed behind a reversed negative, the result being that a brownish image is produced in the bichromate salt. The reverse side of the parchment is then exposed in order to bind it and the film together, and the whole is washed for twenty-four hours to free it from the bichromate. It is next stretched on a frame and covered with glycerine and ammonia, which cause it to swell and become tacky in the parts on which the light has acted least. On passing an inked roller over the picture the shadows take up the ink, but the lights or absorbent parts reject it. Thinner ink is applied to give the half tones. A tracing paper mask is then laid over the film, and the printing paper, which must have a good surface, is laid on it and covered with a sheet of felt, and the whole placed in a copying press and well squeezed. After considerable practice 100 copies per hour can be made.