284 Life and Letters of Francis Galton
superimposition of the portraits, such as I had frequently employed with maps and meteorological traces, that the idea of composite figures first occurred to me'."
This paper by Galton, which I had some difficulty in locating, and of which no copy could be found in his collections, is entitled : " On means of combining various Data in Maps and Diagrams'." Galton therein refers again to his stereoscopic map method', which plan he regrets had not yet been adopted. He says that it needs good models, but that the number of these increases every year, as the then recent French Geographical Exposition demonstrated. He exhibited in the Loan Exhibition stereoscopic views of models taken by the Royal Engineers, but I do not know what subjects they represented. He suggested the stereoscope, not only as a means of showing things in the solid, but of superposing plans and maps for comparative purposes.
He next turns to superposition by means of a telescope ; he remarks that if one half of the object-glass be covered up, the sole effect on the image of a distant object will be to reduce its brilliancy by a half. Now let us suppose two lenses placed one in front of one half of the object-glass and the other in front of the other half, each with its own object at its focal distance ; the two objects will then be combined superposed in the field of view of the telescope. Instead of two lenses four might be distributed over the area of the object-glass to combine four objects, etc. Actually Galton placed his telescope vertically and ran a horizontal tramway under the objective; in the blackened roof of a `tramcar' a number of lenses were inserted and opposite each on the floor of the carriage its own object at the focal distance of the lens. If now lens 1 be brought under the objective we see only object 1, as we push the carriage farther lens 1 tends to pass out of the field and lens 2 to share the field ; thus different intensities of objects 1 and 2 can be combined. A further push of the carriage causes 2 alone to be seen, and the process continued combines it with 3, and so on. Galton's model had six lenses of the same focal distance and size in the roof of his carriage. By this means a series of geographical data which would overcrowd any single map can be combined in sets of two.
"It affords a peculiarly suitable method for picturing changes whether in physical or political geography. I will not describe the mechanism by which complex and powerful instruments of this kind might be constructed ; where the images should be thrown by a lime light on a,screen, and a string of perhaps only three large achromatic collimators would serve for an indefinite number of pictures." (p. 315.)
That Galton should have spoken of the old 'wheel of life' in connection with his apparatus, shows that he had a foreshadowing of the modern cinematograph. By using lenses of different focal length objects at. different distances could be combined, and by using inclined mirrors facing definite parts of the object-glass the objects need not be placed in parallel planes.
' Journal of the Anthropological Institute, Vol. VIII, p. 135, 1878. In this paper (ftn. p. 135) Galton gives 1878, instead of 1876, for the year of, the Map paper.
2 South Kensington Museum Conferences held in connection with the Special Loan Collection of Scientific Apparatus, 1876; Chemistry, Biology, Physical Geography, Geology, Mineralogy and Meteorology (pp. 312-15). London, Chapman and Hall.
3 See our p. 33.