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SCIENTIFIC SIDE-LIGHTS 



but can neither analyze nor define them. 

 Here the refracting prism or the combina- 

 tion of prisms known as the " spectroscope " 

 comes to its aid, teaching it to measure as 

 well as to perceive. CLEKKE History of As- 

 tronomy, pt. ii, ch. 1, p. 161. (Bl., 1893.) 



1943. LIGHT SIFTED BY NATURAL 

 OBJECTS The Color We See Is the Color 

 They Reject. Having unraveled the inter- 

 woven constituents of white light, we have 

 next to inquire what part the constitution 

 so revealed enables this agent to play in 

 Nature ? To it we owe all the phenomena of 

 color, and yet not to it alone; for there 

 must be a certain relationship between the 

 ultimate particles of natural bodies and 

 white light, to enable them to extract from 

 it the luxury of color. But the function of 

 natural bodies is here selective, not creative. 

 There is no color generated by any natural 

 body whatever. Natural bodies have show- 

 ered upon them, in the white light of the 

 sun, the sum total of all possible colors, and 

 their action is limited to the sifting of that 

 total, the appropriating from it of the colors 

 which really belong to them, and the reject- 

 ing of those which do not. It will fix this 

 subject in your minds if I say that it is the 

 portion of light which they reject, and not 

 that which belongs to them, that gives bodies 

 their colors. TYNDALL Lectures on Light, 

 lect. 1, p. 31. (A., 1898.) 



1944. LIGHT, TERRESTRIAL, VA- 

 RIED SOURCES OF Phosphorescence of the 

 Ocean. As in polar light or the electro- 

 magnetic storm, a current of brilliant and 

 often colored light streams through the at- 

 mosphere in high latitudes, so also in the 

 torrid zones between the tropics, the ocean 

 simultaneously develops light over a space 

 of many thousand square miles. Here the 

 magical effect of light is owing to the forces 

 of organic Nature. Foaming with light, the 

 eddying waves flash in phosphorescent 

 sparks over the wide expanse of waters, 

 where every scintillation is the vital mani- 

 festation of an invisible animal world. So 

 varied are the sources of terrestrial light! 

 HUMBOLDT Cosmos, vol. i, p. 202. (H., 

 1897.) 



1945. LIGHT TRAVERSING PURE 

 WATER Undimmed Reflection through Moun- 

 tain Lake. This " gem of the Sierra " [Lake 

 Tahoe] is situated at an elevation of 6,200 

 feet above the sea, and is enclosed in all 

 directions by rugged, forest-covered moun- 

 tain slopes which rise from two to over four 

 thousand feet above its surface. Its expanse 

 is unbroken by islands and has an area of 

 between 192 and 195 square miles. . . . 



On looking down on Lake Tahoe from the 

 surrounding pine-covered heights, one be- 

 holds a vast plain of the most wonderful 

 blue that can be imagined. Near shore, 

 where the bottom is of white sand, the 

 waters have an emerald tint, but are so clear 

 that objects far beneath the surface may 

 be readily distinguished. Farther lakeward 



the tints change by insensible gradation 

 until the water is a deep blue, unrivaled 

 even by the color of the ocean in its deepest 

 and most remote parts. On calm summer 

 days the sky, with its drifting cloud-banks, 

 and the rugged mountains, with their bare 

 and usually snow-covered summits, are mir- 

 rored in the placid waters with such won- 

 derful distinctness and such accuracy of de- 

 tail that one is at a loss to tell where the 

 real ends and the duplicate begins. While 

 floating on the lake in a boat, the transpar- 

 ency of the water gives the sensation that 

 one is suspended in mid-air, as every detail 

 on the bottom, fathoms below, is clearly dis- 

 cernible. 



In experimenting on the transparency 

 of the waters, Professor John Le Conte 

 found that a white disk 9.5 inches in 

 diameter, when fastened to a line and low- 

 ered beneath the surface, was clearly visible 

 at a depth of 108 feet. It is to be remem- 

 bered that the light reaching the eye in such 

 an experiment traverses through water 

 twice the distance to which the disk is sub- 

 merged, or, in the experiment referred to, 

 216 feet. The only instance in this country 

 in which waters have been found to be more 

 transparent is in the great limestone-water 

 springs of Florida. RUSSELL Lakes of North 

 America, ch. 4, p. 64. (Gr. & Co., 1895.) 



1946. LIGHT, "UNNATURAL," IN 

 SOLAR ECLIPSE Hungry Dog Refuses Food 

 Courtiers of Louis XV. I have spoken of 

 the " unnatural " appearance of the light 

 just before totality. This is not due to ex- 

 cited fancy, for there is something so essen- 

 tially different from the natural darkness 

 of twilight that the brute creation shares 

 the feeling with us. Arago, for instance, 

 mentions that in the eclipse of 1842, at Per- 

 pignan, where he was stationed, a dog which 

 had been kept from food twenty-four hours 

 was, to test this, thrown some bread just 

 before " totality " began. The dog seized 

 the loaf, began to devour it ravenously, and 

 then, as the appearance already described 

 came on, he dropped it. The darkness lasted 

 some minutes, but not till the sun came 

 forth again did the poor creature return to 

 the food. It is no wonder, then, that men 

 also, whether educated or ignorant, do not 

 escape the impression. A party of the cour- 

 tiers of Louis XV. is said to have gathered 

 round Cassini to witness an eclipse from the 

 terrace of the Paris Observatory, and to have 

 been laughing at the populace, whose cries 

 were heard as the light began to fade ; when, 

 as the unnatural gloom came quickly on, a 

 sudden silence fell on them too, the panic 

 terror striking through their laughter. 

 Something common to man and the brute 

 speaks at such times, if never before or 

 again; something which is not altogether 

 physical^ apprehension, but more like the 

 moral dismay when the shock of an earth- 

 quake is felt for the first time, and we first 

 know that startling doubt, superior to rea- 

 son, whether the solid frame of earth is real, 



