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252 THE ADVANCE OF PHOTOGRAPHY
while the spaces between these circles are coloured blue. These circles are so small that it takes about 70,000 of them to cover a square inch, yet they are very large compared with the starch grains of the Lumiere autochrome plate, and so, while the Thames screen has the advantage in the regularity and precision of its coloured spaces, the Lumiere scores on account of the minuteness of the coloured particles.
It will be seen that the principle of the action of light upon this plate is precisely the same as that already explained in the case of the Lumiere plate.
When taking a photograph with the Thames plate the coloured screen should be placed in the dark slide so that the film side is turned away from the lens ; the special panchromatic plate should next be placed upon this, so that the film of the plate is on the film of the screen, and then the whole covered with a piece of black paper.
By this " separate " method there is, however, a great gain to the beginner, since, if on development the negative is found to be unsuitable owing to incorrect exposure, a second negative can be taken, and the loss is confined to the price of the panchromatic plate, as the screen can, of course, be used with a fresh plate.
The plates are, however, supplied with the coloured screen and sensitive emulsion on one and the same plate. This is not to be recommended for beginners, since a failure involves the loss of both plate and screen. There is, of course, in this a saving of time for those who are become expert with the plates, since the film being actually attached to the coloured screen, perfect registration must exist between the two.
In the " separate " method the negative must be used to obtain a positive, and then this bound in register with the coloured screen. This is not at all a difficult matter, for on looking at the screen it will be seen that at one end there is a single cross scratched and a double cross at the