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 be more convenient, as well as safer. As a rule, the exposure is from one-third to one-half of that usually required for albumenized paper. In direct sunlight we have found the exposures with different negatives vary from one to ten minutes; in diffused light, from ten minutes to an hour, or upwards. In using the actinometer, it is scarcely necessary to observe that it must be exposed to the same light as the prints, the progress of which it is intended to indicate.

The term development is used for convenience, although it is essentially different from the operation usually known as development, in which the reduction of a metallic salt on which light has acted produces an image, the developer completing or developing an operation which light has commenced. Here light has completed the chemical action; and the operation which follows is a mechanical one, which, by the removal of the sensitive compound where light has not acted, at once makes visible the image, and prevents the further action of light.

As we have seen that the washing away of the superfluous compound must be effected at the side opposite to that which was in contact with the negative, before we can commence development, the tissue must be mounted on another piece of paper with a material which is not affected by water, in order that the paper on which the compound has rested up to the present time may be removed, so as to expose the hitherto protected surface to the water.

As the paper upon which the tissue has to be