Page:EB1911 - Volume 21.djvu/543

APPARATUS] denoting the sensitiveness of their different brands, and is more or less the basis on which the plate-speeds for the modern English dry-plate actinometers and exposure meters are calculated Several systems of photometry and measurement of the speeds of dry plates have been discussed at the meetings of the Congrés International de Photographie, in 1889, 1891, 1900 and 1905, but no definite standard has been finally adopted. In Germany the use of J. Scheiner’s sensitometer has been adopted, and appears to be extending It is based on a system of photographing the graduated tints given by rotating sectors. A full account of the instrument, and of a system of sensitometry based on its use, is given by J. M. Eder in the Photographische Correspondenz (1898) p. 469. and (1900) p. 244. In 1901 Chapman Jones brought out a convenient plate-tester on the same principle as the Warnerke sensitometer, but extended by the addition of a colour sensitometer, which is useful for the comparison of orthochromatic dry plates, colour screens, light filters, &c. It consists of a screen plate, 4×3 in., containing a series of twenty-five tints of graduated densities; a series of coloured squares, blue, green, yellow and red, and a strip of neutral grey, all five being of approximately equal luminosity; a series of four squares of special pure colours, each representing a definite portion of the spectrum; also a space of line design, over which is superposed a half-tone negative. To use the instrument, a quarter-plate of the brand to be tested is exposed behind the screen for a few seconds to the light of a standard candle placed at the distance of a foot, developed, fixed and washed. An examination of the plate will show the sensitiveness, range of gradation, possible range of exposure, sensitiveness to colour, size of grain, amount of halation, and the most suitable light for development. It can be used for many other tests, and enables any brand of plates to be readily tested by the user and compared with any standard he may find convenient. In making these and similar tests, a standard developer should be allowed to act for a fixed period and at a uniform temperature (Ph. Journ., 1901, 25, p. 246).

The next important factor is the actinic power of the light. It depends normally on the height of the sun for the latitude of the lace at the time when the photograph is taken, and exposures in bright sunlight are found to vary approximately as the cosecant of the sun’s altitude above the horizon The light of the sun itself is practically the same at any given time and place year after year, but is liable to more or less local and temporary diminution by the amount of cloud, haze, dust, &c, present in the atmosphere at the time It is also affected by the time of day, increasing from sunrise to noon, and then decreasing to sunset. The remaining factor is the effective diaphragm aperture of the lens in relation to its focal length. In most cases of ordinary outdoor exposures this can be taken at its normal value, but becomes smaller and increases exposure if the focal length is much increased for photographing near objects. Besides these principal factors, the nature and colour of the objects, their distance, and the amount of light received and reflected by them under various atmospheric conditions, have a great influence on the exposure required. W. B. Coventry has shown (op. cit. p. 75) how the “light coefficient L,” for fully sunlight, can be found, and has given a table of values of L for the latitude of London for every hour of the day in periods of ten days throughout the year, also the relative coefficients for “diffused light,” “cloudy,” “dull” and “very dull.” Tables of exposures for different subjects under varying conditions of light have been published by W. K Burton, A. S. Platts, F. W. Mills, Sir D. Salomons and others, and in preparing them Dr J. A. Scott’s tables, showing monthly and daily variations of light for countries about N. lat. 53°, are generally used. The more modern tables, such as are published in the printed “exposure notebooks,” also take into account the plate speeds, but unfortunately there is no uniform standard of plate speeds, owing to the difficulty of fixing a definite standard of light. The subject is fully treated in the British Journal Almanac (1901), p. 675, the Watkins Manual, H. Boursault’s Calcul du temps de pose en photographie, and similar works by A. de la Baume Pluvinel, G. de C. d’Espinassoux and others.

Based on the same principle as these exposure tables, various portable exposure meters have been brought out, in which scales representing the coefficients for plate-speed, light and diaphragm are arranged as in a slide rule, so that, when properly set, the normal exposure required can be found by inspection, and increased or diminished according to circumstances. In Hurter and Driffield’s “Actinograph” the light coefficient is given by a printed card showing the curves for every day in the year and for every hour of the day, the unit being the part of the brightest possible diffused daylight when the altitude of the sun is 90°. The “lens” scale shows the ratios of aperture to focal length in general use, and is calculated for single, double and triple systems of lenses. The “speed” scale is based on the exposure in seconds which with one actinograph degree of light will produce a perfect negative of an ordinary lan scape. An additional scale is given for five different degrees of illumination—“very bright,” “bright,” “mean,” “dull,” “very dull.” A table of factors for “views,” “portraiture,” “interiors,” “copying,” is also given, and these regulate the figure to be taken for the exposure. The scales are engraved on boxwood, and there are two sliding pieces (fig. 68).

It is specially adapted for use with plates of speed numbers agreeing with the H. & D. scale, but can be used with any plate of which the relative speed number is known. Convenient exposure meters have been made since 1890 by A. Watkins, of Hereford, in different forms based upon an actinometrical test of the light at the time of exposure. In the complete “Standard Meter” (1890) scales corresponding to “speed of plate,” “diaphragm f numbers,” “light,” “subject” and “enlarging,” marked P. D. A. S. and E., are arranged on rings adjustable round a cylinder. The plate-speeds are taken from a table and the “light coefficient,” or “actinometer number,”

. 69.—Watkins’s “Standard” Meter. is ascertain at the time exposing a piece of sensitive paper in the actinometer at the end of the instrument for the number of seconds required to match a fixed tint as shown by an attached pendulum. Many improvements have been made in it and the latest pattern (1908) is made in magnalium (fig. 69). The “Dial” meter (1901) is a simpler form in a circular metal case with four apertures marked “plate,” “stop,” “act” and “exp.” above the corresponding scales, and an actinometer for testing the light. The numbers showing the speed of the plate in use, the f value of the diaphragm, and the actinometer exposure in seconds are brought into the respective apertures and the exposure required

. 70.—The Watkins’s “Bee” Meter. is read in the “exposure” aperture. An “indoor meter” is also made, and a “hand camera calculator” for use with the “Standard” or “Bee” meters. The “Queen Bee” and “Bee” meters (1903) are later, smaller and more convenient patterns which have superseded the “Dial” meter and have the plate numbers and exposures marked round the case, and the scales of “f numbers” and “light” on a revolving glass plate. This is revolved till the f number on the right is opposite the speed number of the plate; opposite the “actinometer number” on the left, found as above, will be found the exposure in seconds (fig. 70). The “Queen Bee” meter is similar to the “Bee,” but of better construction and fitted with a pendulum.

G. F. Wynne’s “infallible” exposure meter (1893) is also in dial form, but the sensitive paper is exposed directly, no pendulum

. 71.—Wynne’s “Infallible” Exposure Meter. is used, and the scales are open on the dial. In use, the glass carrying the movable scale is turned until the actinometer time in seconds upon the exposure scale is opposite the diaphragm number of the plate, as given in the list of plate speeds; the correct exposure will then be found against each stop given on the scale. There are practically only two scales: the scale of diaphragms representing the diaphragm apertures or f numbers, the speed of plate and the variation of exposure due to subject; and the time scale representing the actinometer time and the exposure (fig. 71) The actinometer is protected by a yellow glass screen when not in use. In a smaller form the scales are on the 