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the literature of science I read of marvellously delicate instruments devised to make clear to the intellect the mode of operation of forces invisible to the eye, how Alpha rays, Gamma rays, or the vibrations in metal or plant are measured, and I sigh for some device to aid the intellect in solving difficult problems of psychology. I ask myself how may I ascertain with a precision of knowledge which would convince others whether the figures of vision, imagination or dream are two or three dimensional. The figures cast on the screen in a theatre are on the flat, but have all the illusion of motion, distance, shadow, light and form. The figures of human memory I am content to accept as being in two dimensions. They are imprinted by waves of light on the retina, and cast upon some screen in the brain. But I am forced by my own experience and that of