Page:EB1911 - Volume 21.djvu/544

Rh circumference of a locket, and the actinometer at the back. An “Infallible” Printmeter is also made for showing exposures in contact printing on sensitive papers, but can also be used for testing speeds of plates and papers. Beck’s “Zambex” Exposure Meter gives the exposure and stop to be used, also the depth of focus to be obtained with different diaphragm apertures. The required exposure is set to the “speed” number on the next scale of the meter. The third scale; corresponds to the times of darkening the sensitive paper in the actinometer attached to the meter, and shows the diaphragm aperture suitable for the given exposure. Other scales show the distances that will be in focus with the different stops used, arranged so that the focal depth of four different lenses can be found. Several other exposure meters are made on the principle of the slide rule, with scale corresponding to the factors of “plate speed,” “diaphragm number,” “light,” “subject,” “exposure,” and the exposure is found by simple inspection without an actinometer. They are designed for use with particular brands of plates, but can be used for others of similar speeds.

Another class of exposure meters comprises those in which the intensity of the light is estimated visually by extinction through a semi-transparent medium of increasing intensity, such as J. Découdun’s (1888), in which the exposure is judged by the disappearance of a series of small clear openings on a graduated scale of densities when laid on the most important part of the image as seen on the ground-glass. Its indications are not very definite, and the paper scale changes in density after a time. A better form is “E. Degen’s Normal Photometer” (1903), consisting of two sliding violet glass prisms, one adjusted for the diaphragm apertures, the other for the actinic illumination of the object. They are mounted with their outer faces parallel. In use the upper slide with prism is drawn out so that the pointer coincides with the division indicating the diaphragm aperture to be used; the object to be photographed is then viewed directly through openings at one end of the instrument, and the lower slide is drawn out and pushed back slowly till the object viewed is almost obscured. The attached pointer will then indicate the exposure required, or, reversing the order, the diaphragm aperture for a given exposure can be found. Auxiliary scales are attached for very short or very long exposures. The principle of construction is that the logarithms of the times of exposure are proportional to the thickness of the coloured prisms. “G. Heyde’s Actino-Photometer” (1906) is on a somewhat similar principle, and consists of a circular metal box with dark violet glass viewing screens in the centre of both sides, with an obscuring iris inside the case worked by revolving the back of the box. On the front of the instrument exposure tables are given for plates of every rapidity, and for diaphragm apertures from f/3 to f/45. Exposure meters of this type are specially applicable for open-air work where there is sufficient light for ready measurement. Other simple actinometers are in use for carbon and process printing, consisting generally of translucent graduated scales in different densities of paper, coloured gelatin, &c., or of a photographed scale graduated by increasing exposures. The “Burton actinometer,” for pigment printing, made on this principle, contains several small negatives of different densities, one of which is selected of equal depth to the one to be printed, and the progress of the printing is estimated by exposing a piece of sensitive paper under it and examining it from time to time.

Sensitive Dry Plates.—A special feature of modern photography is the use of trustworthy ready-prepared sensitive dry plates and films in different grades of sensitiveness, so that there is ne necessity for the photographer to prepare his own plates, nor, indeed, could he do so with any advantage. The practice of outdoor and studio photography has thus been very greatly simplified; and although with wet collodion there was the advantage of seeing the results at once and retaking a picture if necessary, the uncertainties connected with the use of the silver bath and collodion, and the amount of cumbrous apparatus necessary for preparing and developing the plates, far outweighed it. There is also an enormous saving of time, in using dry plates as compared with wet, by deferring development. In tropical climates, also, dry plates can be used when work with wet plates would be impossible. On the other hand, the uncertainty of more or less random exposures on ready-prepared plates must not be overlooked. Besides their use in taking negatives, gelatin dry plates are also largely used for printing transparencies, lantern slides, enlargements, &c. For negative work they are prepared with an emulsion in gelatin of silver bromide, alone or with the addition of silver iodide or chloride, and are to be obtained in five or six degrees of rapidity: “slow,” for photo-mechanical or “process” work; “ordinary,” for general purposes when quick exposures are not required; “rapid,” for landscape and portraits; “extra rapid,” for instantaneous exposures; and “double extra rapid,” for very quick snapshot work in dull weather or for special subjects. These latter kinds are exceedingly sensitive, and require great care in use to avoid fog. In order to prevent halation, or irregular action by reflection from the back surface of the glass, dry plates are coated with a non-actinic “backing,” which can easily be removed before development.

Self-developing dry plates were introduced in 1906, in which the developing agent is mixed in the film itself, as in the Ilford “Amauto” plate, which only requires immersion in a solution of washing soda for development, or, as in the Wellington “Watalu” plates, applied on the back of the plate, plain water only being required for development, this application also preventing halation. The slow plates used for printing lantern slides and transparencies are usually prepared with an emulsion of silver chloride with or without free silver nitrate and other haloids.

The rendering of photographic plates isochromatic or sensitive to all colours by dyeing them with eosin, or other suitable dyes, has been greatly improved by the use of new dyes, especially those of the isocyanin group, prepared by Dr E. Konig of the Hoechst factory, and known as “orthoclirom T,” “dicyanin,” “inaverdol,” “pinachrom” and “pinacyanol,” the latter of which can confer on a silver bromide plate as high a degree of sensitiveness for red as erythrosin does for yellow; also F. Bayer’s “Homocol,” Dr A. Miethe’s “ethyl red,” and other similar dyes (see E. Jb., 1905, pp. 183, 336). Panchromatic plates are now largely manufactured and used for all photographic work in which a true rendering of the relative colour luminosities is essential, and more particularly for the various methods of colour reproduction in which plates are required to be sensitive to red, green and blue violet. They are made in different degrees of general and colour sensitiveness, according to the purpose for which they are required, the ordinary “isochromatic” being most sensitive for yellow and green, and the “panchromatic” for red, orange and yellow, as well as for green, blue and violet. To obtain the best results from all these plates it is necessary to screen off the blue and violet rays with yellow or orange transparent screens, or colour filters, made of coloured glass, or glass coated with coloured gelatin, collodion, &c., or with glass cells containing solutions of suitable dyes or salts. For the various processes of three-colour reproduction panchromatic plates and special red, green and blue-violet filters have to be used for taking the three negatives, their intensities and absorptions being carefully adjusted to the particular plates in use; the same applies, but less strictly, to the yellow screens used with ordinary isochromatic plates. Dyes specially suitable for these colour-filters have been prepared by Dr E. Konig. Various kinds of colour screens for ordinary, microscopic and trichromatic work are made commercially, and Messrs Schott of Iena make a special yellow glass in three tints for the purpose.

Plates for Colour Photography.—In 1868 Louis Ducos du Hauron, among various trichromatic methods patented for photographically reproducing coloured objects in the colours of nature, described one in which the trichromatic principle, instead of being carried out on three separate plates, was to be combined in one plate by means of a transparent medium covered by a trichromatic screen divided into narrow juxtaposed lines or minute spaces, corresponding to the three primary colours, red, green and blue-violet, the transparent colour of each of these lines or spaces acting as a colour filter. A sensitive panchromatic plate was to be exposed in contact with this screen to produce a negative with lines or spots corresponding to the relative strength of the three coloured lights passing through it, so that a diapositive print on glass properly registered with the tricolour screen would show the object in its proper colours This method could not be carried out successfully for want of efficient panchromatic plates and other difficulties.

Between 1892 and 1898 several patents were taken out by  W. McDonough and J. Joly for various methods of preparing trichromatic ruled screens (Ph. Journ, 1900, p. 191). The Joly method was fairly successful in action, but had several disadvantages owing to the coarseness of the lines, the necessity for having two screens, one for taking and another for viewing, and the cost of making them (B. J. A., 1899, p. 671). The “Florence” chromatic plate (1905), worked out in America by J. H. Powrie and Florence M. Warner, was an improvement on the Joly method, the colour screen being photographically printed on a glass plate, coated with panchromatic emulsion and exposed to the coloured object through the screen (Penrose Pictorial Annual, 1905–1906, p. 111). Some good results were produced, but it has not come into use.

After several years of laborious research, Messrs Lumiere, of Lyons, adopting Ducos du Hauron’s coloured grain method, succeeded where he had failed, and in 1907 brought out their “Autochrome” plates, in a very complete and practical form, making it possible to produce photographs in the colour of natural objects by one exposure instead of three, as in the ordinary three-colour processes. Glass plates are coated with an adhesive medium over which is spread a mixture of potato starch grains, of microscopic fineness, stained violet, green and orange, the interstices being filled in with fine carbon powder to form a tricolour screen, dark by reflected and of a pinkish, pearly appearance by transmitted light. This is varnished and coated with a thin sensitive panchromatic emulsion of gelatino-silver bromide. The plates are exposed in the camera from the back, through the tricolour films, using also a special compensating orange-yellow screen, before or behind the lens, then developed as usual, producing a negative coloured image in the complementary colours, which is then treated and reversed so as to produce a positive coloured image by transmission, showing the picture in its proper colours. The results thus obtained 