3141. To Remove Nitrate of Silver Stains

3141.    To Remove Nitrate of Silver Stains. An inevitable consequence of practicing this process will be stains on the hands and clothing from the nitrate of silver. Moisten the spots with tincture of iodine, and then with a saturated solution of hyposulphite of soda. Cyanide of potassium acts more energetically, but is a very dangerous poison, and is not recommended.

3142. The Photographic Negative or Cliché

3142. The Photographic Negative or Cliché. In number 3128 we have stated the general principles of the photographic art; that it was based upon the fact that solar light decomposes certain combinations of the salts of silver; that in proportion or to the extent that such sensitive surface is exposed to the action of light, so is the depth of the stain or intensity of the image upon the prepared paper. Now if we should cut from an opaque or black piece of paper, any form or figure - an old fashioned silhouette would be a familiar illustration - and place it upon the silvered paper, the precise image or form cut in the paper would, upon removal, be found upon the paper; the paper remaining white under the figure leaf or " theorem," while the parts exposed to the light have turned black. In place of this figure, science has given us the Photographic Negative or Cliché. A negative is an image produced upon glass by a camera (an improved form of the old camera obscura) and derives its name from the fact that the image is reversed or negative by transmitted light (looking through it), the lights appearing dark, and the dark parts light. The chemicals used to produce it are also combinations of the salts of silver, but are so sensitive to the action of light, that they are decomposed instantaneously by exposure. The formulae will follow a description of the process.

3143. To Make a Photographic Negative

3143. To Make a Photographic Negative. In a room illuminated only by a feeble gas or candle light, or by such daylight as is filtered of its chemical power through a sheet of yellow glass, a glass plate is carefully flowed with collodion. (See No. 3149 (Collodion for Photography).) When the plate has been evenly covered, the excess is quickly but deliberately returned to the bottle, and the plate gently and slowly swayed from side to side until the collodion is set, or when the surface is tacky to the touch. It is then placed on a dipper, and, with a steady, continuous motion, immersed, collodion side upwards, in the silver bath, (See No. 3150 (Silver Bath).) If the plate is stopped in its descent into the bath, a check or line will show across its face. In 3 to 5 minutes, depending upon temperature, etc., the plate is coated, or, in other words, the chemicals in the collodion have united with the nitrate of silver, forming the sensitive surface or coating, If not coated sufficiently the surface will appear greasy; in this case the plate must be returned to the bath until the film appears perfectly smooth. While this is being done it is supposed that the operator has adjusted the camera upon the object to be photographed by focussing his lens. This is done by turning the lens in and out, or from and towards the ground glass of the camera, until the point is ascertained which gives the sharpest image upon the ground glass. All being ready, the operator returns to the dark room for his sensitive plate. This is placed in a "holder," and the ground glass being removed, the holder is substituted in its place. The slide or cover to the holder is now withdrawn and the sensitive plate exposed to the action of the image of light thrown upon it by the lens. After an exposure of 15 to 60 seconds, depending so much upon the intensity of the light that it can only he ascertained by experience, the slide is replaced in the holder and the plate taken to the dark room for development.