SUNLIGHT, SEA, AND SKY. 59 



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 strength- 

 ened 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 con- 

 structed with proper means of registering these phenomena with ac- 

 curacy 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 pho- 

 tographer notices that he can obtain the most perfect image in his pict- 

 ure, 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 preeminently to investigate this phenomenon that the last 

 Eclipse Expedition, furnished with funds by her Majesty's Govern- 

 ment at the instance of this British Association, was sent out. And 

 upon this investigation all the powers of the twin instruments of mod- 

 ern times, the spectroscope and the polariscope, were turned. The 



