But the principle should be laid down, viz., that every variety of sensitive paper corresponds with an optimum ratio of highest and lowest opacities for the negative, and that no negative can give equally satisfactory prints on papers having different characteristics.
As it is impossible always to obtain negatives of the same character, except when working under strictly identical conditions, it is well to have two or three varieties of paper of different gradations at one's disposal, and to choose the paper which suits each negative best. The judicious choice of paper is one of the essential elements of success in photographic printing.' This rule does not apply when printing on transparency plates of any kind. The latter can be used with very varied types of negatives by suitably regulating the exposure and time of development. For this reason transparency plates are not supplied in various degrees of contrast 2 as in the case of papers.
504. Printing Frames and Machines. Printing complete protection of the latter from any light other than that transmitted by the negative, and to prevent any relative displacement of the two surfaces, which would produce a double image. Further, in the case of sensitive paper, etc., on which the image appears during ex posure to light, one should be able to note the progressive " building-up " of the picture at any time, so that the printing can be stopped at the right moment.
The chief advantage of a printing box over a frame is that the dark-room is not illuminated with white light during printing. In this way many precautions which would otherwise have to be taken are dispensed with, and several operators can then work at the same time. The usual type of printer, in which the negative is illuminated by diffused light, is not suitable for printing when making reproduced negatives or transparencies on rigid supports (positive plates). Defects in the plane surface of the glass never permit of two plates being in complete contact over their whole and for this reason a perfectly sharp print can only be obtained if we use a small source of light, fixed in position relative to the frame and far enough from the latter for it to be considered as a point source of light.
sos. In France the standard type of printing frame for print-out papers is the so-called French frame, represented diagrammatically in Fig. 180. A piece of white plate glass, about in. to in. (6 to 8 mm.) thick and free from hubbies and scratches is placed in a hard wood frame with deep rebates. The negative to be printed is then placed face upwards on the glass support, then the printing paper, the sensitive surface in contact with the film of the negative (with or without a mask between), then a 1 L111,1, or spongy rubber (failing this, several thicknesses of soft tissue paper) ; and, lastly, the back, con sisting of two or more hinged panels, is placed in position. The pressure-bars are then closed and fixed in position.
With a frame of this kind, the state of the picture can be ascer Tamed Dy disengaging one of the pressure-bars, opening the corre sponding flap, and gently lifting the pad and the paper. 2 Various simplified types, called English or American frames, for sizes up to 7 x 5 in. (13 x I8 cm.), are not fitted with plate glass, or only with thin plate or ordinary glass if they are used for printing film negatives. Their inside dimensions should then be only about in. (I mm.) larger than the negative. As the bars of such frames are usually fixed in position by turning them in a plane parallel to the back, there is always a risk of moving the back, and consequently the paper on which it is resting. Any risk of movement can he prevented if the pins of the hinges extend from the back into grooves in the frame.
A special type of frame, in which the back is fixed to the frame with well fitting hinges, is used in the few cases when the sensitive layer is coated on a rigid support (transparencies, or opal print-out plates), or again in cases where the support of the sensitive layer is too soft to be replaced in exactly the same position on the negative after it has been lifted up for examina tion (silk and other sensitized fabrics 9. The negative is fixed in the frame, and the sensitized material on the back. Similar frames have been used, which allow of a transparent or trans lucent film (§ 516) being interposed between the negative and the sensitive paper during printing.