In the reproduction of a screen-plate it is clear that we might place it with the film in contact with the sensitive plate. Then we should have the color elements reproduced quite sharp, and every negative would be broken up into minute dots corresponding in size. For some positive processes this might not be a disadvantage, but in the case of the separate system in which the positive is bound up with a viewing screen, we cannot obtain contact with the sensitive surface. The result will be more or less want of sharpness, but this will not be of serious moment, and it breaks up the screen pattern, so that almost closed or continuous tone negatives are obtained. Actually it is advisable to follow the same plan with an autochrome, that is, to interpose between the sensitive surface and the picture film a colorless transparent medium of greater or lesser thickness.
There are some very simple mathematical formulas by which we can tell not only what ought to be the thickness of the intervening medium, but also the distance of the light source, and the resulting want of sharpness in the resulting negatives. But one of the main ideas in this little book has been to avoid, as far as consistent with clarity, mathematics and deep theory, so that it will be assumed that it is required to make the negatives from autochrome and Paget pictures. A camera may be used and it should be racked out to 50 cm. If this is not possible, then the simple device given on p. 169 should be used, and it will be assumed that it is required to enlarge the screen elements so that they just close up. Then the size of the light-source is found by the following simple rule:
Size of light =
Element-size X Enlargement - 1 X Camera Extension
The degree of enlargement to cause the image to coalesce is taken as 4, the camera extension is set at 500 mm, the separation thickness is that of the autochrome, which is approximately 2 mm, and the size of the element is taken as 0.025 mm, so that we have:
(4 - 1) X 500
Now the aperture in the box, where the complementary filter was placed, and which can now be replaced by the sharp-cutting filters, is actually the source of light, therefore, we merely have to cut a card with a circular aperture of 18 mm in diameter and place over it a ground glass to obtain the required effect.
In the case of the Paget plate, we may take the size of the color element as 0.115 mm, and the thickness of the viewing screen as 0.5 mm. Then we have:
(4 - 1) X 500
If this size is too large to be convenient, and it might be if the camera were used, then the thickness of the glass, or the distance between the two films, is found from the simple equation:
(Size of En-
Size of Light
(largement - 1) X Size of Element. So, if we set the size of the light at 50 mm, then:
When using the camera, the lens should be removed and the ground glass put in its place, with an opaque card of the correct aperture. If the aperture is larger than the lens flange, then the lens board may be removed, which is usually possible.
Magnesium ribbon will be found the most convenient light-source. It may occur to the reader that this is really a very small source of light: but actually the ground glass becomes the light, and it is easy with magnesium to evenly illuminate such a relatively small area. Daylight may be used, and the camera should then be pointed direct to a white cloud or a white sky, not a blue sky.
It is unnecessary to enter into any further particulars than have already been given as to the printing processes. The same rules apply, and the so-called complementary colors must be used for printing.