The screen-plate idea was originated by Louis Ducos du Hauron as long ago as i868, but no practicable screen was made until 1895, when Prof. John Joly, of Dublin, produced screens formed of fine lines of colour in juxta position. These screens were, however, so costly to manu facture that the Joly process was commercially a failure ; and it was not until 1907 that the first really successful screen-plate was placed on sale, by Messrs. A. & L. Lumiere, under the name of the Autochrome Plate. The introduction of this plate marks a new era in colour photography. From being a costly and lengthy process, involving many delicate operations with proportionately numerous risks of failure, photography in natural colours became at once so simple and inexpensive as to be within the reach of almost every amateur.
The Autochrome Plate.—The Autochrome screen is com posed of millions of grains of potato-starch, of an average diameter of about T,3u-ff in. The method of manufacture is, briefly, as follows. The starch grains, separated into three lots, and dyed respectively red, green, and blue-violet, are intimately commingled until the mixture appears of a uniform grey tint. The glass plate having been coated with a transparent tacky varnish, the coloured grains are "dusted on," until the whole surface is uniformly covered.
The grains are then rolled into close contact, no interstices being left, and a protective varnish is applied. The screen is then coated with a panchromatic emulsion, and the plate is complete.
The mode of using the plate is as outlined above, but it should be noted that, in this and all other screen-plate processes, a yellow " compensating screen " is used before or behind the lens, to cut off the excess of blue and violet rays.
Since the arrival of the autochrome plate, a bewildering number of methods of screen-plate manufacture have been patented by different inventors, and many varieties of screen plates are now on the market. Space will only allow of our referring in detail to those which are most generally in use.
The Omnicolore Plate.—The Omnicolore plate, though not yet actively marketed in this country, possesses a special interest as the actual invention of the veteran pioneer of colour photography, Louis Ducos du Hauron, in conjunction with his nephew, R. de Bercegol. Its introduction immediately followed that of the Autochrome plate, in 1907. In differs from the latter in the formation of the screen, which consists of a film of gelatine, dyed in a regular pattern of red and green rectangles crossed by blue-violet lines. The red rectangles have an area of about 4-6 x in. ; the green ones are somewhat larger, and the width of the blue lines is about in. The dyed gelatine
being more translucent than starch grains, the time of exposure is proportionately shorter than in the case of the autochrome plate.
The Thames Plate.—The Thames plate is a British production, the inventor being Mr. C. L. Finlay, of London. In this plate also the screen consists of dyed gelatine, the colour units being a regular series of dots of red and green, each about in. in diameter, the interstices being coloured blue-violet. This screen is particularly translucent, and as the emulsion is also a rapid one and its colour sensitiveness so adjusted that the compensating screen need only be of moderate depth of colour, comparatively short exposures may be given with successful results. A feature of the Thames plate is that the screen may be had either with or without the sensitive coating. In the latter case it is used in conjunction with a pan-chromatic plate, the two being placed together in the dark slide, film to film. 'They are separated again before development, and the plate may be fixed as a negative and any number of positives printed from it, each of which, when bound up in register with any Thames screen, becomes a complete colour photograph. This method of reproduction is, of course, only possible with a perfectly regular screen, such as the Thames.
The Dioptichrome Dioptichrome plate, another French product, is the invention of Monsieur Louis Dufay, and was first issued in 1909. It is now readily procurable in this country, and the results shown are so excellent that a wide popularity may be predicted for it. In structure the screen is somewhat similar to the Omnicolore (described above), the pattern consisting of continuous green lines, interspersed with rows of red and blue rectangles. The colour-rendering of this plate appears to be exceptionally good ; flesh tints and white (the latter a very severe test for a screen-plate) being reproduced with remarkable fidelity.
It will have been noticed, from our general description of the screen-plate method, that there is no superimposition of colours. The colour sensation is not produced by the subtraction, from the light falling upon the photograph, of one set of colour rays after another. The rays which pass through the photograph to the eye are of the exact colours of the light-filter elements—red, green, and blue-violet—and it is by the addition of colour ray to colour ray in the eye itself that the correct colour sensations are produced. This is known as the additive principle, in contradistinction to the subtractive principle of the superimposition processes before described.