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.
Conzpensating Screens.—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.
Development.—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 : Red.-50o c.cm. 6 per cent. gelatine, grammes dark room red dissolved in too c.cm. water.
Green.-500 c.cm. 6 per cent. gelatine, 4 grammes dark room green in u00 c.cm. water.
Dark Red.—The same proportions as for red.
7 c.cm. dyed gelatine will coat mo square centimetres of glass.
Messrs. Newton and Bull advise the following screens : 1. Tartrazine . r mgram.) per sq. centimetre of glass.
Rose Bengal . „ 2. Methyl Violet . . „ 1/ 11 /1 The proportion practically comes to Yu and gr. respectively to the square inch of glass. The red screen transmits light from the end of the visible red to X 5,9oo in the yellow. The methyl violet absorbs from X 6,400 to X 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.