Various devices enable colour plates or films to be examined under suitable conditions. We may mention the wallet-like viewing cases and the illuminated frames.
The viewing cases have a frame, with adapters in which the plate or film (mounted between two sheets of glass) can be placed in a plane inclined at about 45", and connected by a bellows to another frame containing a mirror placed hori zontally. The observer looks in the mirror within this kind of box with black walls, and sees the reflected image of the plate or film illuminated by daylight or by a lamp.
The illuminated frames have at the back of the frame receiving the transparency, a diffusing surface (white paper, matt, opal glass, or aluminium roughened by a sand-blast) illumin ated sideways by tubular-shaped lamps hidden from the view of the observer.
When a colour screen plate or film is lighted or projected by artificial light, it is often of advan tage, at least in a room which has no other light, to absorb the excess of red in the light used by means of a very slightly bluish filter, of which the depth must be ascertained by trial.
projection of colour-screen plates or film requires a very powerful light-source. As these images absorb a great deal of light, they become heated very rapidly and would soon be destroyed through softening of the varnish which holds the coloured starch grains in position. Each plate must therefore remain only a short time in the beam of light.
As far as possible, the practice of alternating colour and monochrome slides (hiring a lecture must be avoided. The latter are far more trans parent, and cause a dazzle effect, which makes the colour slides following them appear much darker. If it is impossible to avoid showing monochrome slides, they may be bounded with old colour-screens from which the silver image has been removed, thus making them darker. It is, however, simpler to show first of all the monochrome slides and then the colour-screen image or vice versa, the power of the light being reduced for projecting the monochrome slides.
891. Monochrome Negatives from Colour Screen Transparencies. It may sometimes be necessary to obtain black and white prints, same size or enlarged, of a subject of which only a. colour screen transparency is available. A nega tive may be made by contact printing, prefer ably on a panchromatic plate with a yellow filter balancing about equally the lights trans mitted by the three kinds of starch grains in an area corresponding to a white part of the image. But (especially when enlargements are
made) such a negative always shows somewhat marked granularity in the images of objects in bright colours, owing to the masking of the grains of one or two colours.
This granularity is avoided and at the same time the outlines arc softened by copying the transparency in a camera by the Artigiie two plate method (312), the composite negative thus obtained from an autochrome being capable of enlargement up to 5 times (L. 1923). The granularity of a contact negative may also be reduced by copying or enlarging it by one of the methods described in § 313.
892. Duplicate Colour Screen Transparencies.
For the reproduction of images with a regular or irregular tricolour mosaic on similar plates and films it is advantageous to use specially prepared originals, denser and more contrasty than those best suited for direct viewing or projection, obtained by normal exposure and a somewhat curtailed first development. The difficulties of reproducing colour-screen tran sparencies are due on the one hand to the structure of the image, and on the other, to the faulty saturation of the colours.
Let us take, for example, the image of a red object. Oa a certain area of this image there may be, for instance, 300 uncovered red grains and 600 masked grains (.loo green grains and 300 blue grains). If a sharp image of this plate is obtained on a second plate of the same nature, it is probable that owing to the distribution of the grains according to the laws of chance, of the 300 red grains only too will be projected on to the red grains in the second plate ; the remaining 200, being projected on to green and blue grains, will not be recorded, and the final image will thus be composed of too red grains and Soo black grains, so that it is considerably darker than the original image. This darkening is easily remedied by introducing a certain degree of diffusion into the images (use of an incompletely corrected lens or imperfect focus sing when copying is done in the camera ; faulty contact and use of a comparatively large source of light, when copying is clone by contact printing).