The difference thus measured between the two densities is equal to the logarithm of the grada tion of the paper, a characteristic which it would be very desirable to see makers indicate on each packet. The instructions for some densitometers give average values for some papers in current use ; it is easy to determine it by testing a sample from each batch under a calibrated sensitometric screen.' Knowledge of the maximum density, or at least of the greatest density requiring considera tion, permits the optimum printing exposure to be chosen. If we know in well-defined condi tions of illumination (intensity of light source and its distance from the sensitive paper) the exposure necessary with a given paper, with a given development, to obtain a very light grey scarcely different from white, under a known density (e.g. under a sensitometric wedge), then we can calculate the exposure required for the same grey under any other density (other condi tions being the same), the ratio of the two exposures being the number whose logarithm is the difference between the two densities considered.
If, for instance, the optimum exposure was 12 seconds under a density 1-5, the exposure under a maximum useful density 1-9 will be obtained by multiplying the exposure of the test (it would be necessary to divide if the new density were lower than that of the test) by the number 2-5, which is the anti-logarithm of o-4, the difference between the two densities compared. Hence the new exposure is 12 X seconds, or 30 seconds.
The calibration data or their practical applica tion (type of paper to be used and regulation of the exposure counter or rheostat) should be written in one of the margins of the negative.
512. Modes of Printing. A negative may be printed in solid, the paper being entirely covered by the picture (at least after trimming), or printed under a mask, so as to leave narrow or wide white or tinted margins, or, again, vignetted, the tones of the image passing imperceptibly from their full intensity to the white or tinted base of the paper.
It is always possible to improve certain parts of the picture relatively to others during print ing by local variations either of the illumination or of the exposure. In this way the effect of working-up the negative can be supplemented, or dispensed with altogether.
There are various methods by which too great a sharpness of the image can be avoided, the effects of a sharp and diffused print being proportionally combined as required (§ 313), while excessive contrasts in a photograph can be lessened by breaking-up the uniformity of the masses of shadows.
Images can be combined in various ways, such as the introduction of a background or sky after the usual printing, surrounding the picture with a toned border, or in other ways.
513. Contrivances for Printing. The masks used to protect the sensitive paper during print ing are generally made of black paper free from pinholes or thin parts, sometimes of very thin red celluloid, or sheets of tin-foil. Celluloid has an advantage in that it can be adjusted more easily on the negative, the image being visible through the mask. Foil is seldom used except in certain processes of photo-mechanical repro duction where the print is on a rigid support. It can be used very much thinner than celluloid or even paper, and there is less risk of blurring being caused by bad contact.' Cut-out can be bought in various shapes and sizes—rectangles with rounded corners, ovals, circles ; also fancy shapes, the use of which is not always pleasing. Such masks are usually supplied with their corresponding opaque counterparts or counter-masks. The counter-masks can be employed to obtain black margins on a print which has been already fully printed, by means of a second printing, the print being exposed under the counter-mask.
A mask may also be cut out with a penknife to any required shape, or, if a rectangular opening with sharp corners is required, it can be made by sticking strips with clean-cut edges on to the gelatine surface of the negative itself.
When printing from film negatives, the mask may be inserted between the negative and the glass of the frame instead of between the negative and the sensitive paper.
In photographic establishments where ama teur's work is undertaken, it is the practice to supply the print with a narrow white margin, without, however, any trouble being taken to select the best opening or the most suitable position for the mask on the negative. In such cases a frame mask or border mask is generally used. One side of the mask is covered with a thin card, cut out to the size of the negative, which is thus automatically centred on the opening. When a border mask is to be used for printing on large-sized paper or card, e.g. for printing small negatives on post cards, the paper may be adjusted more easily in the opening if another cut-out card is used, or clearly visible register marks are made on the existing 94. Printing with a Tinted Border. Pleasing effects may be obtained by surrounding a picture which has been printed on a larger size of paper with variously-sized borders or edgings in grey of different intensities. These effects can be obtained by means of a combination of masks carefully cut and accurately marked in relation to the sensitive paper.