Life and Letters of Francis Galton
By November 27, 1900, Galton had devised a simple small apparatus'
' This is now in the Galton Laboratory. It consists essentially of a triple camera; one object is thrown directly on to the focal plane; the other two by aid of two reflecting prisms are also thrown on to the same plane. The three objects thrown on to the plane are positive a, negative a and positive b. By throwing out one or two of the three we can throw on to the screen (i) positive a, (ii) negative a, (iii) positive b, (iv) positive a + negative a to show the uniform grey, (v) negative a + positive b, the transformer, or (vi) positive a + the transformer, giving the darkened b.