Blue Methylene (4 per cent. solution) . . . so min.
. . . . . . . . 5o „ Green Methylene (4 per cent. solution) . . 3o min.
Auramine G. „ It . i8o „ Orange Erythrosine a per cent, solution) . . . 35 min.
Metanile Yellow (sat. solution) at 6o° Fahr. . 4o „ and these solutions may be adapted for liquid filters, dry filters, or for staining the plates themselves.
But the question of filters is scarcely one on which it is wise to dogmatise. Three different classes of filters are in common use for colour work, and all seem, in the hands of skilful operators, to produce excellent work : (I) Filters that divide the spectrum into three bands, from some point in the red to D, from D to F, and from F to H, forming three zones without overlap or gaps.
(2) Filters forming three zones, which considerably over lap each other.
(3) Filters whose zones contain gaps or lacune.
The first system is the older one and might almost be called the orthodox method, but the second has very strong support and is rapidly gaining favour.
Exposures for Landscape.—The ordinary landscape dark slides are unsuitable for three-colour work, owing to the impossibility of securing exact registration of the three negatives. No two dark slides can be depended upon to hold the plates in precisely the same relative position. During the withdrawal and replacement of a slide the camera is certain to shift, and, however slight this movement, it will destroy any hope of getting the negatives to correspond. A " repeating back " must be employed comprising three screens of the proper colours, which fit on to a dark slide containing one long, narrow plate, on which the three ex posures are successively taken, a spring catch denoting when each third of a plate is in position. There are difficulties in gauging the relative exposures for the three screens, which may vary from 4 : 2 : I to 3 : 2 : according to the type of plate and the intensity of light. One great difficulty about these three exposures in the open air is that, owing to atmospheric changes, the light may vary in quality. If one plate is taken in clouded weather and the next in sunshine, the results will be inharmonious. The exposures must also be such that plates of corresponding density are obtained ; and they must be accurate within 2 per cent.
By the way, the student may be interested in knowing that Herr Stenger has worked out the relative exposures with ordinary non-colour-sensitised plates, using, of course, the proper filters. They are, blue 1, green 750, red 9,00o.
The ultimate results are, he contends, exactly similar to those obtainable on panchromatic plates.
One-Exposure Cameras.—Obviously, a system which will give all these exposures simultaneously is to be preferred. But the triple-lens system, as hitherto applied, is unsuitable for the photography of near objects. The images from the three lenses side by side will each represent an image, vary ing only slightly, perhaps, yet sufficiently to make coincidence impossible with regard to any object nearer than one hundred times the focal length. Herein comes the advantage of the Sanger Shepherd one-lens, one-exposure camera, in which, by means of prisms, the three exposures are given at one time, and relatively through each screen. Only two sizes are at present to be obtained to give negatives x 2in., and about lantern size ; but, of course, it is easy to make enlarged negatives from these on ordinary slow plates. The method relieves the operator from all anxiety, as there is not the slightest danger of varying exposure, or of the register being lost by any movement of the camera before the three negatives have been secured.
Development is not a very serious matter, if care be taken not to work too near the source of light, and to use a movable cover (a cardboard-box lid will do) for the developing dish. The plate becomes less sensitive as develop ment proceeds ; great density should be avoided, and even ness of development is important.
Lantern Slides.—According to the older fashion, three transparencies in black and white were made from the three negatives, and projected by a triple lantern through red, green and violet glasses. This system is now replaced by the printing of three coloured positives on thin celluloid ; and when these are bound together, and projected through the ordinary lantern, they should reproduce with fair accuracy all the colours of the original. Lantern slides projected by either method, when carefully executed, far surpass in brilliancy anything hitherto effected by means of the screen plates.