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 These, however, are points of detail in which each operator will adopt the method which suits him best. It is assumed that the negatives will be sorted and classified; and, where it is necessary, the printing qualities of each, and the section of screen, or number of unit tints, required on the actinometer will be ascertained by one or two preliminary trials, and marked on the negative. With a simple system of classification and registration it will be easy to secure sufficiently uniform results. In some large printing establishments a similar system is pursued in silver printing. The negatives are classified, and the whole of one class being exposed at one time, it is only necessary to examine the progress of printing under one negative, which becomes practically an actinometer; and when it is completed, it is known that all the others, possessing like qualities, and having been submitted to similar conditions, are completed at the same time.

The sensitive paper for the actinometer may be prepared by almost any formula, provided uniformity be observed. Plain Saxe paper, immersed for ten minutes in a ten-grain solution of chloride of sodium, may be kept ready for use. This, when required, may be floated for two minutes on a forty-grain solution of nitrate of silver, and will be found to answer every purpose.

The apparatus is supplied with the necessary mechanical appliances for ready change and examination of the sensitive paper, and is found perfectly practical, being at once easy to use, and efficient for the purpose for which it is designed.

The photometer or actinometer used by Mr. Swan, and described above, was found to be wanting some of