CALOTYPE. The patent for Mr. Fox Talbot's Calotype, obtained in 1841, contained specifications relating to many departments of photographic operation ; but the proper calotypic process is what we are here con cerned with. The operation comprises three stages—to prepare the paper ; to produce the negative picture ; and to produce the positive picture. The best and smoothest writing paper is washed on one side with a camel hair pencil dipped in a solution of 100 grains of crystallised nitrate of silver in six ounces of distilled water. The paper is dried slowly, and then dipped for a minute or two in a so lution of 500 grains of iodide of potassium in a pint of water; it is then dipped in water and dried. Immediately before using, this paper is washed on the prepared side with a solution of 100 grains of nitrate of silver in two ounces of distilled water, combined with one-fourth of its volume of acetic acid, and an equal quan tity of tincture of galls or of a saturated solu tion of crystallised gallic acid. The paper is dried a third time, and is then ready for use. In taking the calotype picture, the paper is placed in a camera obscure, where it speedily receives the image, which sometimes requires to be brought out by a wash of gallo-nitrate of silver. To fix the image, the paper is dipped into water, partly dried with blotting paper, and washed with a solution of 100 grains of bromide of potassium in 8 or 10 ounces of water, after which it is again washed and dried. The image or picture thus pro
duced is a negative one, the lights and darks being reversed. To produce a positive picture from this, a second sheet of calotype paper, or of common photographic paper, is placed in contact with the picture ; a board is placed beneath, and a plate of glass above them, and all are screwed closely together. On placing them in the sunshine for some time, a picture with the lights and shadows in their natural posi tion is produced on the second paper ; and this duplicate is to be fixed in the same man ner as the original picture. Many calotype positive pictures may be taken from one negative picture.
In another method, Mr. Talbot produces a positive calotype at once, without the inter vention of a negative one. A sheet of calo type paper is exposed to the daylight until its surface is slightly browned. It is then dipped into a solution of iodide of potassium ; on being taken out of this solution the paper is dipped into water and partially dried. It is next placed in the camera, where, in the course of five or ten minutes, the image be comes impressed upon it. The paper, when removed from the camera, is washed with gallo-nitrate of silver, and warmed. A posi tive picture then makes its appearance on the surface.