In place of Nitrate of Silver, recommended in the formula, Chloride of Silver may be used, but not Iodide of Silver, as the formation of Iodide of Sodium would be objectionable (p. 136). For the same reason it is better not to add any part of the Hyposulphite Bath used for fixing Negatives, to the Positive colouring solution. This toning Bath is not to be employed immediately after mixing, but should be set aside until a portion of Sulphur (produced by free Hydrochloric Acid, and Tetrathionate of Soda reacting upon the Hyposulphite) has subsided. It will be very active at the expiration of a few days or a week; but upon keeping for a longer time, loses much of its efficacy by a process of spontaneous change. The immersion of prints also lessens the quantity of Gold; and hence, when the Bath begins to work slowly, more of the Chloride must be added, the Sulphur being allowed to deposit as before. Filtration through blotting-paper will not be required. The writer finds that after a certain time, when the Bath has been long used, and organic matters, Albumen, etc., have accumulated in it, it is better, and more economical, to throw away what remains, and to prepare a new solution. The addition of Chloride of Gold to an old Bath will not always make it work as quickly as one recently mixed. THE MANIPULATORY DETAILS OF PHOTOGRAPHIC PRINTING. These include--the exposure to light, or printing properly so called; the fixing and toning; and the washing, drying, and mounting of the proof. The Exposure to Light.--For this purpose reversing frames are sold, which admit of being opened at the back, in order to examine the progress of the darkening by light, without producing any disturbance of position. Simple squares of glass however succeed equally well, when a little experience has been acquired. They may be held together by the wooden clips sold at the American warehouses at one shilling per dozen. The lower plate should be covered with black cloth or velvet. Supposing the frame to be employed, the shutter at the back is removed, and the Negative laid flat upon the glass, Collodion side uppermost. A sheet of sensitive paper is then placed upon the Negative, sensitive side downwards, and the whole tightly compressed by replacing and bolting down the shutter. This operation may be conducted in the dark room; but unless the light be strong, such a precaution will not be required. The time of exposure to light varies much with the density of the Negative and the power of the actinic rays, as influenced by the season of the year and other obvious considerations. As a general rule, the best Negatives print slowly; whereas Negatives which have been under-exposed and under-developed print more quickly. In the early spring or summer, when the light is powerful, probably about ten to fifteen minutes will be required; but from three-quarters of an hour to an hour and a half may be allowed in the winter months, even in the direct rays of the sun.