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Nominal Outside

slides, colour, lantern, slide, §, plates and negatives

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NOMINAL OUTSIDE DimENsioNs Continental Europe (Fig_ 195) 85 x 100 mm.(3.35 x 3-95 ins.

Great Britain (Fig. x96) 31 x 31 in. (82-5.x 82-5 min.) United States (Fig. 3} x 4 in. (82-5- x 'or ma.) In principle, the two rectangular sizes must always be used as shown in the figures, the hori zontal lines of the subject being parallel to the longer side of the plate.

A good deal of latitude has been introduced in the standardization of lantern-slide sizes since colour plates with a multicolour screen have come into general use. These plates afford direct positives which it is rather difficult to duplicate, so that, for showing them, all the usual hand-camera sizes, up to 9 X 12 cm. (31. X 4 in.) inclusive, have been admitted, and many recent projection apparatus allow images of 9 X 12 cm. size being projected, and are fitted with carriers taking the pictures either vertically or hori zontally. On the other hand, to permit of the projection of slides made from stereoscopic negatives, special carriers have been made which allow one half of a stereo slide to be shown. We cannot, however, too strongly advise the photographer wishful of showing his slides anywhere without difficulty to adopt the most generally-used size. Otherwise he may be com pelled to take with him not only his slides, but also all or part of the appropriate projection apparatus.

780. Photographic Lantern Slides. Lantern slides are usually made on special plates yielding images of black tone (§§ 567-569) or of warm tone (§§ 574-578), with the facility of toning or dyeing (§§ 595-606). They are sometimes also made by various pigment processes (§§ 655 and 674 to It is advantageous to vary the colour of the slides of a set, so as to avoid monotony, but it is necessary to avoid too vivid colours in striking contrast with the average colour of the subject.

When charts, or numerical tables are shown in alternation with views, etc., in tone, the excessive brightness of these subjects, with extensive areas of white, is so dazzling as to prevent the next views from being seen satisfactorily. Such alternations of semi-dark ness and light may be fatiguing. To prevent this, it is often preferred to project the negative," instead of a positive, of the line subject. Such

negative must not be too dense. In order that the with which the lecturer points out details on the screen, may be visible, it is advis able that such negatives be toned to a warm colour.

For the printing of lantern slides by contact there are various types of special printing frames allowing the printing aperture to be centred over one part or another of a larger negative. There are also available commercially reduhere cameras (similar in construction to box-foon enlargers) for copying by daylight on a redurm scale from large negatives. Reduction from negatives of very large size (such as X-ray plates) is usually done in a triple-body camera with an arrangement for illuminating the negative with diffused light.

While, with some practice, it is usually possible to judge satisfactorily the colour and 781. Diagram and Notice Slides. To save time, a notice or simple drawing may be made direct on the slide without the aid of photo graphy.

The notices can be written or typed on thin paper, gelatine, or Cellophane, inserted between two sheets of " carbon paper " as used for obtaining carbon copies. In this way two identical impressions are obtained, one on each side, and these reinforce each other.

The writing or drawing may also be done with Indian ink on first-class tracing paper, or with pencil on ground glass ; or, again, it may be done by scratching with a stylus on a glass previously prepared by coating with one of the varnishes used by copperplate etchers.

The slide may also be made by writing with contrast of a lantern slide by holding it at an angle of 45° in front of a white background which is uniformly illuminated (care being taken to avoid reflection of light from the image), it is always well to test the slides by passing them through the lantern in the normal manner. This precaution is specially necessary in the case of slides which have been toned to another colour than black ; while they may appear perfect when viewed direct, they may sometimes cause an unpleasant surprise when projected.

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