Page:Popular Science Monthly Volume 2.djvu/71

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form. The interior parts have consequently undergone strains and pressures in different directions and in different degrees, in accordance with which each part has become the subject of a definite internal molecular arrangement; and these, by each in its own way, modifying the light which they transmit, give rise to the figures now before you.

I will conclude this series of experiments by one which, although not so beautiful or striking as those which you have already seen, is still interesting as bringing the subject home to us, and as the only application of polarization to commercial life which has yet been made. You will recollect the brilliant sequence of color shown by a quartz plate when submitted to polarized light. Well, the effects produced by that quartz plate are also produced by not only some other crystals, but, what is very remarkable, also by many of their solutions, e. g., by that of sugar. Into this tube I have put a solution of sugar; when it is placed before the lamp, polarization colors are shown on the screen, while the liquid itself remains colorless. If the solution be strengthened by the addition of more sugar, the tints vary; and, by accurate observation of the colors for different positions of the Nicol, the strength of the solution may be determined. An instrument constructed with proper means of registering these phenomena with accuracy is called a saccharometer.

These experiments may be multiplied almost indefinitely, and many a long winter evening might be spent in following polarization into other branches of science upon which it has something to say. For example, on examining a variety of vegetable and animal tissues, slices of wood, fronds of fern, scales of fish, hair, horn, mother-of-pearl, etc., with a suitable polariscope, we should find that they exhibit, internally, definite structural characters, capable of affecting the light, which they transmit in the same general way as do crystals. Or again, if we were to apply the principles established in an early part of this lecture to the conditions of sky, aspect, and time of day under which the photographer notices that he can obtain the most perfect image in his picture, we should find that they correspond with those which will furnish him with daylight in the most perfectly polarized condition.

Once more, among the many and curious phenomena which are visible during a solar eclipse, there is one which has longer than any other refused to lift its veil to the solicitations of science. I mean that halo of light, or corona as it is called, which extends beyond the dark disk of the moon, beyond those red flames of burning gas which the researches of Lockyer, of Janssen, and of others, have brought almost home to us, far away for millions of miles into distant regions of space. It was preëminently to investigate this phenomenon that the last Eclipse Expedition, furnished with funds by her Majesty's Government at the instance of this British Association, was sent out. And upon this investigation all the powers of the twin instruments of modern times, the spectroscope and the polariscope, were turned. The