SWAN'S ACTINOMETER.
Before proceeding to manipulatory details, it is desirable to mention the mode of meeting another serious difficulty which has hitherto stood in the way of carbon printing. As the film of bichromated gelatine and carton is black, there is no darkening during exposure to indicate the progress of printing. No perceptible change is made in the film by the action of light.
In the uncertainty arising from this circumstance it has been necessary to guess the exposure, or estimate it by a consideration of the strength of the light and the 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.
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.