density of the negative. If carbon printing were to be carried out on an extended scale, it was manifest that some more accurate mode of proceeding was necessary. Mr. Swan and Dr. Vogel have met the want by the instruments described below.
Swan's actinometer consists of a small box, in which is inclosed a piece of sensitive paper, carefully screened from the action of all light except that to which the operator submits it. This box is provided with a sliding lid, in one aperture of which is fixed a small screen of glass, which has been collodionized, excited, exposed, developed, etc., so as to form a miniature negative, nearly opaque at one end, and nearly transparent at the other. Under a small section of this (of an appropriate degree of translucency), the sensitive paper is exposed to light. Another portion of the lid consists of yellow glass, underneath which the sensitive paper can be pushed, and examined without danger of injury from the light; the slightest tint of the portion upon which light has acted being readily distinguishable through the yellow glass from the white portions upon which light has not acted even. It also possesses an arrangement for bringing under the screen for exposure successive portions of the sensitive paper, as each colored portion has done its office.
The actinometer, in its most perfect form, is provided with the graduated screen, or a series of screens, each of different density, corresponding to the density of various negatives. But it may be very easily worked with one screen, in which case the screen is very translucent and is termed a unit screen; with this, several repetitions of a constant tint—and that almost the first remove from absolute white—are given in each printing—two, three, four, or more, according to the density of the negative.
