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Page:Popular Science Monthly Volume 54.djvu/321

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a table a figure (Fig. 4) made of light cardboard, fastened to blocks of wood at the base so that the pieces would easily stand upright. The middle piece, which is rectangular and high, was placed a little in front of the rest of the figure. The students were asked to describe

PSM V54 D321 Halftone process on film creating an illusion.png
Fig. 7.—This is a highly enlarged reproduction taken from a half-tone process print of Lord Kelvin. It appeared in the Photographic Times.

precisely what they saw, and with one exception they all described, in different words, a semicircular piece of cardboard with a rectangular piece in front of it. In reality there was no half-circle of cardboard, but only parts of two quarter-circles. The students, of course, were well aware that their physical eyes could not see what was behind the middle cardboard, but they inferred that the two side pieces were parts of one continuous semicircle. This they saw, so far as they saw it at all, with their mind's eye.

There is a further interesting class of illustrations in which a single outward impression changes its character according as it is viewed as representing one thing or another. In a general way we see the same thing all the time, and the image on the retina does not change. But as we shift the attention from one portion of the view to another, or as we view it with a different mental conception of what the figure represents, it assumes a different aspect, and to our mental eye becomes quite a different thing. A slight but interesting change takes place if we view Fig. 5 first with the conception that the black is the pattern to be seen and the white the background, and again try to see the white as the pattern against a black background. I give a further illustration of such a change in Fig. 6. In our first