I now remove the analyzer, and put in its place the piece of Iceland spar with which we have already illustrated double refraction.  The two images of the carbon-points are now before you, produced, as you know, by two beams vibrating at right angles to each other.  Introducing a plate of quartz between the polarizer and the spar, the two images glow with complementary colours.  Employing the image of an aperture instead of that of the carbon-points, we have two coloured circles.  As the analyzer is caused to rotate, the colours pass through various changes:  but they are always complementary.  When the one is red, the other is green; when the one is yellow, the other is blue.  Here we have it in our power to demonstrate afresh a statement made in our first lecture, that although the mixture of blue and yellow pigments produces green, the mixture of blue and yellow lights produces white.  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.  Through this glass the beam from the polarizer now passes, being intercepted by the Nicol in front.  On exciting the magnet light instantly appears upon the screen.  By the action of the magnet upon the heavy glass the plane of vibration is caused to rotate, the light being thus enabled to get through the analyzer.

The two classes into which quartz-crystals are divided have been already mentioned.  In my hand I hold a compound plate, one half of it taken from a right-handed, and the other from a left-handed crystal.  Placing the plate in front of the polarizer, I turn one of the Nicols until the two halves of the plate show a common puce colour.  This yields an exceedingly sensitive means of rendering visible the action of a magnet upon light.  By turning either the polarizer