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PHOTOGRAPHY IN NATURAL COLOURS 259
in which the dark lines are double the width of the transparent intervening spaces. It has been found practicable to use such a grating with 600 lines per inch, hence much finer than anything attempted in the Joly process. AYhen being used to prepare a screen, the grating is placed in the printing-frame, and the sensitive plate laid in contact with it. Exposure is then made to an arc light. The plate is then removed, developed in the usual way, and the line images in the insoluable colloid dyed up with green colouring matter. In this way one-third of the surface of the plate has been covered with green lines, and twTo-thirds still remain transparent. The plate is again coated with sensitized bichromated colloid, and the grating arranged so that the lines of the grating are in register with the green lines ; and great care is required for this, since the lines of the grating are double the width of the coloured lines on the plate. After exposure and development as before, the red colour is applied to the plates.
The blue is finally added by a repetition of these methods, except that exposure is made through the glass of the screen, and then on developing and dyeing up it is found that the blue automatically fills up the remaining spaces. This screen is on the market, and is known as the Florence Heliochromic Screen Plate. This screen plate is coated with a panchromatic emulsion, and thus a means is provided of obtaining a coloured photograph by making an exposure, just as would be done in the case of any orthochromatic plates.
Two varieties of plates are to be provided ; those for obtaining negatives in colour will have the lines running in a lengthwise direction, while those intended for the production of coloured positives are to have lines running parallel to the shorter side of the plate, thus enabling one to automatically place the plates in the correct position when obtaining a colour positive from a colour negative.