By enlarging our aperture, the two images produced by the spar are
caused to approach each other, and finally to overlap. The one image
is now a vivid yellow, the other a vivid blue, and you notice that
where these colours are superposed we have a pure white. (See fig. 43,
where N is the end of the polarizer, Q the quartz plate, L a lens, and
B the bi-refracting spar. The two images overlap at O, and produce
white by their mixture.)
[Illustration: Fig. 43.]
Sec. 8. _The Magnetization of Light._
This brings us to a point of our inquiries which, though rarely
illustrated in lectures, is nevertheless so likely to affect
profoundly the future course of scientific thought that I am unwilling
to pass it over without reference. I refer to the experiment which
Faraday, its discoverer, called the 'magnetization of light.' The
arrangement for this celebrated experiment is now before you. We have,
first, our electric lamp, then a Nicol prism, to polarize the beam
emergent from the lamp; then an electro-magnet, then a second Nicol,
and finally our screen. At the present moment the prisms are crossed,
and the screen is dark. I place from pole to pole of the
electro-magnet a cylinder of a peculiar kind of glass, first made by
Faraday, and called Faraday's heavy glass.
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