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Photographic Researches and Portraiture   313

with the aid of Mr T. R. Dallmeyer for carrying out his project, and this was exhibited on the date just mentioned to the Royal Photographic Society; Galton's paper is printed in The Photographic Journal, Vol. xxv, pp. 13538. The idea at the bottom of Analytical Photography is extremely simple, as most of Galton's methods. A subject A and a subject B, taken in similar positions and of similar size, have faint transparent positives and faint transparent negatives taken of each. If now positive A and negative A be thrown accurately adjusted on the same screen, they will antagonise each other and give a uniform grey background. If further positive A and negative B be thrown on .the same screen, they will only antagonise one another where the originals are identical; where they are different, they will only in part antagonise each other. Thus the combination of positive A and negative B gives a representation of their difference on a grey ground. This Galton calls the "transformer." If the transformer be thrown on the screen with positive B, it converts positive B into positive A. Similarly negative A and positive B is the transformer, which superposed on positive A, converts it into B. The two transformers are in fact positive and negative of the same difference. In both cases the transformed portrait is that of a darkened subject. The fact that our combination of faint positive and faint negative gives a uniform grey or half tone is very important; because it follows that where our transformer adds nothing in the way of difference to A to make B, it will still add everywhere this grey or half tone. The transformed B will therefore be a darkened picture of A.

Galton illustrated this point by obtaining a `real' scale of tints. He took nine teetotums: the first had a white surface, the second a sector of 45° painted black, the third two sectors of 45° black, the fourth three sectors and

Diagram v. Galton's photograph of a spinning wheel of tints.