LANTERN SLIDES Transparent positives, for projecting by means of the optical lantern, are made in England to a standard size of 3+ in. square. Specially prepared slow plates are used, the object being to secure a fine transparent quality in the image, combined with good rendering of gradation and cleanliness in working. A good slide should be very transparent ; that is, there should be an absence of any tendency towards clogging or opaque appearance ; the deepest shadows should be strong so as to give a rich colour when pro jected. There should be scarcely any part abso lutely white or free from deposit, and the range of gradation should be as perfect as possible, with full detail in both light tones and shadows.
Plates of two kinds are used. One corresponds in speed and character with the papers used for developing in weak gaslight, while the other is similar to a slow bromide paper in speed, hut differs materially in character. The first kind will be considered later. For the second kind a yellow light is the most satisfactory when deve loping, etc., and there is a wide range in speed, the faster varieties yielding black tones only, while the slower ones give readily any tone from warm brown to black by simple modifica tions in exposure and development. These plates are sufficiently rapid for producing slides by reduction in the camera by daylight or arti ficial light, using a condenser in the latter case ; or they may be employed with equal facility for contact printing by artificial light. An ordinary camera is employed for making slides by re duction in daylight.
Any adjustable camera may be used, provided that it is not smaller than 3+ in. square. There must be a frame to hold the negative parallel with the sensitive plate, and at a suitable distance in front of the lens. In adjusting the distances of the various parts of the apparatus, the same proportion of the focal length of the lens will be required as in enlarging, but with this differ ence : the greater distance will be that between the lens and the negative ; the smaller, the dis tance from the lens to the sensitive plate on which the image is produced. The space be
tween the lens and the negative must be covered in so as to exclude as much extraneous light as possible, or the slides will suffer considerably, both in gradation and brilliancy. The apparatus should be pointed towards a window, preferably one that commands a clear sky view, in the same manner as in enlarging by daylight.
In making slides by reduction in an artificial light enlarging apparatus, the only difference in setting will be the distance from the lens to the easel and the extension of the camera body, these distances being the same proportion of the focus of the lens as in enlarging by daylight. With some enlarging lanterns in which the source of light is an incandescent gas mantle there is much greater risk of an image of the mantle being pro jected on to the easel in reducing for slide making than in enlarging. This can be entirely pre vented by interposing a piece of ground glass between the condenser and the light.
Definite data for exposures cannot be given. With the slowest of the plates suitable for camera reduction, the exposure may range from fifteen seconds on a clear spring day, using f/i6, and a thin negative, when working for black tones, up to as much as eight times as long for warm colours. Strong negatives will require much longer, and some of the more rapid plates will be fully exposed with one-fourth of these times. For artificial light, as described, these exposures would be correct if f/8 were the lens aperture used.
For contact printing, magnesium ribbon forms the best illuminant, especially when warm colours are desired. The colour and quality of the light influence the colour produced by development. For black tones, i in. of ribbon at a distance of 6 ft. will be sufficient for a medium or thin nega tive, or 4 in. at 3 ft. will yield a good warm tone.