The objection to the use of Nitric Acid is, that if allowed to come in contact with the dress, it produces stains which cannot be removed unless _immediately_ treated with an alkali. A drop of Ammonia should be applied to the spot before it becomes yellow and faded. When Positives are to be taken, it is advisable to use additional care in preparing the glass, and especially so with pale transparent films and neutral, Nitrate Bath. After a glass has been once coated with Collodion, it is not necessary in cleaning it a second time to use anything but pure water; but if the film has been allowed to harden and become dry, possibly dilute Oil of Vitriol or Cyanide of Potassium may be required to remove stains. When glasses have been repeatedly used in photography they often become at length so dull and stained, that it is better to reject them. COATING THE PLATE WITH THE COLLODIO-IODIDE OF SILVER. This part of the process, with that which follows, must be conducted in a room from which chemical rays of light are excluded. It is inferred therefore that the operator has provided himself with an apartment of that kind. The most simple plan of preparing the room is to nail a treble thickness of yellow calico completely over the window, or a part of it, the remainder being darkened. To this a single thickness of a waterproof material made by coating linen with gutta-percha may be added as a further security against the entrance of white light, the smallest pencil of which admitted into the room would cause fogging. It is often convenient to illuminate by means of a candle screened by yellow glass. A dark orange yellow, approaching to brown, is more impervious to chemical rays than a lighter canary-yellow. Lamps suitable for the purpose are sold by the manufacturers of apparatus and chemicals. Before coating the plate with Collodion, see that the fluid is perfectly clear and transparent, and that all particles have settled to the bottom; also that the neck of the bottle is free from hard and dry crusts, which, if allowed to remain, would partially dissolve and produce striæ upon the film. In taking small portraits and stereoscopic subjects, these points are of especial importance, and every picture will be spoiled if they are not attended to. A useful piece of apparatus for clearing Collodion is that represented in the following woodcut. [Illustration] The Collodion, having been iodized some hours previously, is allowed to settle down and become clear in this bottle; then by gently blowing at the point of the shorter tube, the small glass siphon is filled, and the fluid drawn off more closely than could be done by simply pouring from one bottle to another.