Photographic Researches and Portraiture 315
and negative a have the same tone, so that if b were a, the composite would give merely medium grey. It is needful also that pos. a and neg. a should accurately antagonise one another. Our figure shows one of the illustrations Galton gave at the lecture, namely the "transformer" which will change an I into a G.
G + & F
TRANSFORMED F + TO G+
Galton also transformed a mosaic St George's Cross into a St Andrew's Cross (slides in Galtoniana). It may be said of these results that they were only partially successful-this is the view taken by Sir George Darwin in his account of Galton in the Dictionary of National Biography (Second Suppl. Vol. ii, p. 72). But Galton recognised the main difficulty himself: