The Yellow Filter.—But although by the use of suitable dyes the silver salts are rendered much more sensitive to green, yellow, and red rays, there are still a vast number of rays impinging on the plate and being absorbed by it at the opposite end of the spectrum. Dr. Kenneth Mees and Dr. Sheppard are agreed that, if no glass is used, practically two-thirds of the whole effect on the ordinary plate is due to the ultra-violet rays, and if a sheet of glass is used, about one-third. Messrs. Newton and Bull have shown that the ultra violet rays are even more effective with enclosed arc rays and wet collodion. Such rays are most disastrous for colour rendering. This light is quite invisible to the eye ; its effect on colour cannot be gauged, and it entirely falsifies the tones. In a recent lecture before the Royal Photographic Society, Mr. R. W. Wood exhibited a number of photographs taken by these invisible rays, when Chinese white appeared as black. Incidentally they have their value in detecting certain chemical and other pheno mena unseen by the eye, but in pictorial photography we would rather be without them. Hence the yellow filter, which may be introduced either as a lens cap in front or behind the lens, or in the diaphragm slot.
The Yellow Screen for Landscape.—Even with an ordinary plate the yellow screen will give better renderings of land scape 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 6o c.cm. alcohol or methylated spirit is added. The solution keeps indefinitely in the The plates to be sensitised are carefully dusted and bathed in the dark for about three minutes in the dyc 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. i 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.
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.