308 Life and Letters of Francis Galton
wasted too much time already on what may be of no use. So I simply send the enclosed. Abney used a rotating cylinder with a black and white drawing wrapped round it, in order to get the photographic equivalent to each combination of black and white*.
with patter round it
What good news about Pearl. I return his letter. I will shortly send the centile paper and diagram. Some delay has necessarily occurred about it. I am not idle but get through things now so slowly.
Best Christmas wishes to you all, and may you enjoy cake with the F. G. cut. My Nature of last week has miscarried so I have not yet seen my own paragraph, though others, like yourself suspecting me, have written to me about it. The post is just going out so I conclude now,
Ever affectionately, FRANCIS GALTON.
To obtain the mean tint of a rectangular picture.
.Mount the picture on a [rotating] cylinder with axis vertical [?horizontal], in front of a camera. The dark slide of the camera to have a narrow vertical slit. Take an exposure-then cap. Move screen the width of the slit and take a second exposure; again cap. Repeat the process until the sum of the widths of the slits is equal to the length of the picture. Print off. The print will be streaky and of same width as the original was long. Mount the print crossways on the same cylinder as before and proceed as before. The result will be a plate of a uniform
tint, the mean tint of the original. F. G. Dec. 25, 1906.
Galton's plan to get a mean tint is suggestive although it is not quite clear how he proposed to carry it out in practice, especially in dealing with the relative mean tints of engravings, say, of different sizes, or of piebald skins. Would a whole series of cylinders be needfull to fit subjects of different heights, or must the subjects first be reduced to a standard size by photography ? How in practice would such reduction affect the relative tints of the two engravings? Again I do not follow the necessity for the slit, or how it is to be moved A photograph of the engraving on the rotating horizontal cylinder would "give vertical streaks on the plate. A print from this, which must be taken under stringently standardised conditions, could then be put crosswise on .a cylinder of proper size and again photographed to obtain a uniformly tinted negative and thence a print. The "greyness " of this print would have-with absolutely standardised conditions-some relation to the average tint of the; engraving, but I cannot see that they would be the same. Supposing the lighting always (artificially) -the same and the exposures identical, it would be possible to compare: the "greyness " of the prints thus defined with those obtained from known amounts of black and white on the cylinder, and thus form a scale. As I have said, Galton is here suggestive, and the problem is of some practical importance, but it needs much experimental work before it can be considered solved.
(13) Work and Correspondence of 1907. The year 1906, owing to reasons in part indicated, had been a year of stress and change for both Galton and
A similar arrangement was adopted in the Biometric Laboratory for tint comparison judgments in 1894 (see Phil. Trans. Vol. 186 A (1895), p. 392). It is still used in the Anthropometric Laboratory attached to the Biometric Laboratory.