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CH. X] PHOTOGRAPHIC ENLARGEMENTS 379
meters (14 in.) in diameter. Of course, if only a part of the negative plate contains the picture to be enlarged then a smaller condenser in the given case will answer. The above figures are for the diagonal of the respective sizes. These condensers are usually of relatively long focus, especially those of the larger diameters, e. g., the 35 cm. lens ordinarily has a focus of 50 centimeters. The condensers most used for enlarging are usually of the double form, the convexities facing each other as for the magic lantern condenser (%. 185)
§ 534. Objectives to use for printing. — It is necessary to use an objective which has been corrected for photography. The ordinary projection objective gives a good visual image, but not a good photographic image, hence it is better to use a photographic objective.
§ 535. In focusing, some white paper should be put into the printing frame or pinned in place and the image focused with care. The photographic paper when put in the same place will then give a sharp picture.
§ 536. Photographic paper for printing with projection apparatus.— If one has sunlight or the arc light the developing papers like Velox, Cyco, etc., are plenty rapid enough. If a weak light is all that is available, then Haloid or one of the more rapid bromide papers will be called for.
§ 537. Holding the paper while printing. — (A) If the pictures are of microscopic objects or other pictures of relatively small size (i. e., up to 30 x 35 cm.; 12 x 14 in.), a good method is to put a clear glass in a printing frame and press the printing paper down upon it just as one does for printing from a glass negative by contact. This holds the paper perfectly flat and ensures uniform sharpness. With the printing frame one can lay it flat if a mirror or prism is used, or it can stand on edge facing the objective if no mirror is used.
(B) If the printing paper is large the usual method is to have a board screen on a track. The picture is then got of the desired size by varying the distance between the board and the objective,