Erythrosin 0.1 g.
Tartrazin 1.0 g.
Distilled water 1000 ccm.
Bathe three minutes and dry without washing, although the plate should be just rinsed. Instead of tartrazin, naphthol yellow may be used in about the same strength. The exact quantity of the yellow dye will depend on the results desired. The more used, the more the blues and violets are cut out, and the worse they will be rendered in the final picture.
The filter to isolate the red region can be made with yellowish eosin 2 g and tartrazin 4 g per square meter, but Wratten & Wainwright's No. 22 or E2 is suitable. This, of course, must be the same size as the plate used, and the rear plate must be a panchromatic plate, preferably backed. Instead of staining up the front plate, a filter can be placed on the lens; this can be made with 1 g of filter yellow per square meter, or either a Wratten K1 or K1 1/2 can be used; the former gives better rendering of the blues.
There is no need to give instructions as to the development of these plates; but a caution may be necessary as to the front one. It may show rather more contrast than is desirable as compared with the rear one, and further it will probably not stand such a strong developer, and should be developed separately in this case.
If one decides to use the tri-pack system, then the front plate should be a chloro-bromide or transparency, but as it has to record the blues it should not be stained, and a yellow filter on the lens, such as a K1, will be efficient. The second element has to be the recorder of the greens and should, therefore, be orthochromatic, and as has already been pointed out the smaller the thickness of this the better. Therefore a film should be used, and the ordinary roll film may be adopted. As a filter we must use one that cuts out the blue and violet but transmits the rest of the spectrum undamped; it is obvious that we cannot use here the normal green, as this cuts out the red, so we must use a yellow such as K2, which has about the right cut. Behind this middle element we must use a filter that isolates the red, and the normal tri-color red filter may be used (see p. 29). Wratten & Wainwright No. 25 or A is correct.
The tri-pack will, therefore, be composed of the following, counting from the front plate which faces to the rear: a chloro-bromide plate with glass towards the lens; a Ki 1/2 filter; a roll film or orthochromatic film with the film towards the lens; a standard tri-color red filter; a panchromatic plate with the sensitive surface towards the lens.
So far it has been pretty easy sailing, on paper, and the requirements are definitely fixed and fairly easily satisfied; but we now come to the crux of the whole matter, and that is how to adjust the sensitiveness of the plates and the filter actions so that the plates will all require the same exposure. The only thing to be done is to decide this by trial and error, by photographing a scale of greys with various combinations of plates until the scale is rendered alike on all three members. Alteration of the filters is practically excluded, because this would alter the absorptions and upset the color rendering. The most that one can do with the filters is to cut down the light with black and this is not an easy matter.
The tri-pack system outlined above is designated as the dialyte system. as Ducos du Hauron called his suggested arrangement a "polyfolium dialyticum." But J. W. Bennetto, in 1897, suggested a semi-dialyte system, in which one plate receives one image direct, and two plates at right angles to the former receive the other two images. This simplifies matters considerably, and there is no reason why this system should not be more generally adopted. A suitable camera is not on the market, but it would not be a difficult matter to make one. The principle is shown in Fig. 27, in which a represents the camera front, C is a reflector at an angle of 45 ° which throws some of the light to a2 where is placed a bi-pack of two sensitive plates with their surfaces in contact with a filter D1 in between, while D is the filter for the single plate. Obviously this can be turned upside down and the two plates placed at the bottom of the camera. There is here a choice as to the division of the plates, but personally the writer would place the red filter plate at D and the transparent chloro-bromide plate at a2, using a tri-color green filter between this and a panchromatic or orthochromatic plate. In this case, the reflector back must be coated with a minus green color, such as fuchsin or eosin, and the remarks on the construction of the camera on p. 52 should also be noted as to the thickness of the filters, etc.
In this system, the choice of plates and the determination of those most likely to give equal exposures is considerably facilitated, as one can definitely fix on a panchromatic for the red and green records, thus leaving only the chloro-bromide for the blue as the variable factor, and one variable is much easier to solve than two. One has, therefore, far less work in adjusting the relative speeds, because the speeds of the panchromatics may be so chosen as to be of great assistance in adjustment. One can here use a filter on the lens and thus avoid staining up a plate, and the standard tri-color filters may be used.
There may be some little trouble in registering the plates, because of the distortion from the reflector, but the notes on p. 55 will be of assistance. The registration can be readily tested, as by drawing a square with diagonal lines of a goodly size and photographing down it will be easy to see if the three negatives will register. If not, it is easy to determine which one is faulty and make the corrections accordingly. It may even be necessary to distort the reflector slightly by packing the edges with metal or hard rubber strips, but when once satisfactorily adjusted there should be no need to tamper with it again.
It is obvious that in all these processes, as one of the plates is placed with its glass to the lens, the image is reversed laterally; but this is actually an advantage as, if the pictures are used for lantern slides, this can be made on a separate glass and used as the cover glass.