Transparencies are made by the regular photographic processes of exposure and development, on glass plates or films placed in contact with the negative, or in the appropriate position in an enlarging camera. The sensitometry and the terms used to describe the qualities of plate or film for this purpose are those already given in connection with the general discussion of plates and films. But the kind of emulsion to be selected is quite different from the aerial negative emulsion. There is here no practical limitation to the speed, contrast or latitude. Consequently, we can choose a positive emulsion on which the exposure through the aerial negative falls entirely on the straight line portion of the characteristic curve, thus reproducing all of its tones, and the contrast of the negative may be increased to any desired extent. The possibilities of positive emulsion are indeed rather greater than the usual aerial negative can utilize. A range of clearly graduated opacities of two or three hundred to one is possible, so that not only can detail be well rendered in the high-lights, but also equally well in dark shadows where, indeed, an increase of illumination is necessary for it to be made easy to examine. This range is to be contrasted with the l-to-7 range in the aerial landscape, which may be doubled by a contrasty plate. In resolving power, the positive emulsion, which is slow, exceeds the negative emulsion. It easily bears examination through a magnifying glass, thus making any enlargement unnecessary in the printing process.
Glass transparencies are of course impractical for general distribution, on account of their fragility. Heavy film transparencies are not open to this objection, and, especially in the form of stereos, constitute the most beautiful fcrm cf aerial photographic print.