Even with an ordinary plate the yellow screen will give better renderings of landscape effects, such as clouds floating on a blue sky, autumn tints, sheaves of corn, etc., etc. Some years ago we used a simple form of this light-filter, recommended by Mr. G. T. Harris in his little work on landscape photography, and it is a very serviceable one. It was made by dissolving 15 gr. of gelatine in 2 oz. of cold saturated solution of ammonium picrate; this was coated on two circles of flat cover-glass. When dry the two circles were cemented together with Canada balsam. If the slab on which the glasses are coated is inclined slightly, much of the liquid gelatine will flow towards one end. We can thus produce an extra filter for use when photographing landscapes in which the sky effects are worth preserving. This picrate-yellow filter does not interfere with colour values, except by subduing the violet; the exposure is about doubled.
Deep filters must be employed with caution in landscape work. They are very serviceable for dispersing mist on wet days and for afternoon work in cities. But the exposures are necessarily long, and there is the danger of over-correction of colour-values. A field of yellow corn may appear in the print as white as a snowdrift.
Many of the firms dealing in photographic apparatus supply sheets of gelatine stained for the purpose of making yellow filters. A single layer will double exposure, two layers require about four exposures, and so on. The best light-filters are made by staining gelatine with rapid filter yellow K. The making of such filters is now understood much better by manufacturers than in former years, and satisfactory patterns may be obtained at nominal prices. Such filters should, however, be tested, not only as to the extent they influence exposure, but as to whether they are made on optically correct glass or affect the focus. According to Sacco, the angle between the two glass surfaces must not be greater than one minute.
It must not be supposed, however, that any kind of yellow filter is suitable for use with all kinds of orthochromatic plates. Each needs its own particular filter to produce the best possible results, just as in the newer processes of actual colour photography. For this reason some makers are supplying plates which do not require any screen, a layer of easily soluble dye upon the surface of the film (which will disappear during development) acting as the light-filter. Dr. E. Koenig advises the bathing of ordinary dry plates in the preparation known as Erythrosine-filter-yellow. One gramme erythrosine-filter-yellow is dissolved in 120 c.cm. distilled water, and 60 c.cm. alcohol or methylated spirit is added. The solution keeps indefinitely in the dark.1 The plates to be sensitised are carefully dusted and bathed in the dark for about three minutes in the dye solution, then drained and placed in a perpendicular position to dry. The drying must take place in a perfectly dark room without ventilator. As many as twenty-four quarter plates may be bathed in this solution, and the bathed plates generally keep better than unbathed plates of the same brand. The general sensitivity is reduced by about one-half. Plates made by the boiling method are not, however, very suitable for this treatment.
The above plates give a fair rendering of pale greens, orange, and yellow. For reds it is necessary to substitute one of the two new pinorthol dyes, No. 1 of which contains pinachrome, and No. 2 pinacyanol in combination with yellow. With all these plates it is quite unnecessary to use a yellow filter on the lens. But there is some dispute among practical workers as to whether these dyed plates give satisfactory effects, as compared with orthochromatic plates and the screen.
1 Or, separating the dyes, the same results may be attained with filter-yellow 7.7 gr., erythrosine 1.5 gr., water 21 oz., spirit 10 1/2 oz.
It is not necessary to give very elaborate instructions as to when an orthochromatic plate is proper or the reverse. For landscapes of all kinds it is a decided improvement; but when the atmosphere is misty the worker must decide whether he wants mist or absence of mist in his picture, because the screened ortho-plate will ignore it more or less. At eventide, when the light is of yellow tinge, the filter is not needed, but on dull days the greens of the woodland are obviously improved by its use.
The advantages of ortho-plates over ordinary plates for marine pictures and seaside effects are very great, and the yellow screen must be employed to compensate for the intense illumination and the omnipresent blue rays. For the delicate colouring of flowers and fruit, as well as for fabrics and in copying paintings, the ortho-plate follows as a matter of course, the yellow screen being less necessary if the source of the illumination be gas or oil. For instantaneous work, even, it will frequently commend itself, and for interiors when there is much colour and time can be spared for a very long exposure. The chief point to be noted is that an ortho-plate is more useless than any other if under-exposed, but it will bear a tremendous amount of over-exposure.
For some special departments of work it is sometimes found advisable to employ colour-compensation screens of various tints, in order to secure the maximum amount of contrast. The rule, which will, however, admit of exceptions, is to photograph a coloured object through light of approximately the same colour, if contrast within its own structure is desired. But if the main purpose is to obtain contrast with adjacent objects of other colours, then it should be photographed through light of complementary colour.
In the development of orthochromatic plates no unusual difficulty is likely to occur. Nearly every developer is suitable, provided that the worker is sufficiently experienced in it to keep matters under control. Density comes more quickly than with ordinary plates, and the object of colour-gradation might be lost if the negative is allowed to get too hard. The light of the dark room must, however, receive attention. Deep ruby might serve with care, but the ruby light is most unpleasant to the eyesight and very difficult to discern objects by. Dr. Koenig advises a combination of red and green in two separate sheets of glass, the gelatine sides facing each other. Such light is not only safe, but is most agreeable to the eyes. The proper dyes may be obtained, and are mixed for use as follows :
500 c.cm. 6 per cent. gelatine, 4.5 grammes darkroom red dissolved in 100 c.cm. water.
500 c.cm. 6 per cent. gelatine, 4 grammes darkroom green in 100 c.cm. water.
The same proportions as for red.
7 c.cm. dyed gelatine will coat 100 square centimetres of glass.
Messrs. Newton and Bull advise the following screens :
Tartrazine . . . 1 mgram.
per sq. centimetre of glass.
Rose Bengal . . 1/2 "
Methyl Violet . 1/2 „
" " "
The proportion practically comes to 1/10 and 1/20 gr. respectively to the square inch of glass. The red screen transmits light from the end of the visible red to A, 5,900 in the yellow. The methyl violet absorbs from X 6,400 to 5,000, leaving only the extreme red, to which the best panchromatic plates can only be very feebly sensitive.
For further particulars on this subject we must refer the reader to Dr. Koenig's Natural Colour Photography, translated into English by E. J. Wall; The Photography of Coloured Objects, by Dr. Kenneth Mees, D.Sc.; or Colour Correct Photography, by T. T. Baker.