Rfift. 



lity 



SCIENTIFIC SIDE-LIGHTS 



708 



knowledge of Nature is true, so will his 

 work represent the thoughts which have 

 power to charm, instruct, and better man- 

 kind through all time. I know of no better 

 example of the complete reconcilement of 

 poesy and an accurate knowledge of Nature 

 than is contained in Tennyson's " Two 

 Voices." Let any one watch the birth of 

 a dragon-fly, and say whether or not the 

 poet has written sweetly and well and all 

 the more sweetly because his words are true: 



To-day I saw a dragon-fly 



Come from the wells where lie did lie. 



An inner impulse rent the veil 



Of his old husk : from head to tail 



Came out clear plates of sapphire mail. 



He dried his wings: like gauze they grew; 



Through crofts and pastures wet with dew 



A living flash of light he flew. 



ANDREW WILSON Science and Poetry, p. 

 11. (Hum., 1888.) 



3504. TUTELAGE PROLONGED BY 

 CIVILISATION More Time Needed to Pre- 

 pare for the More Complex Life. Now it is 

 owing to the necessity for having a certain 

 number of the more useful routes estab- 

 lished before the babe can be trusted from 

 its mother's side that the delay of infancy 

 is required. And even after the child has be- 

 gun to practise the art of living for itself, 

 time has still to be granted for many pur- 

 poses for new route-making, for becoming 

 familiar with established thoroughfares, for 

 practising upon obstacles and ingredients, 

 for learning to perform the journeys quick- 

 ly and without fatigue, for allowing acts 

 repeated to accelerate and embody them- 

 selves as habits. In the savage state, where 

 the after-life is simple, the adjustments are 

 made with comparative ease and speed; but 

 as we rise in the scale of civilization the 

 necessary period of infancy lengthens step 

 by step, until in the case of the most highly 

 educated man, where adjustments must be 

 made to a wide intellectual environment, the 

 age of tutelage extends for almost a quarter 

 of a century. DRUMMOND Ascent of Man, 

 ch. 8, p. 287. (J. P., 1900.) 



3505. TWINKLING OF STARS EX- 

 PLAINED Interference of Light-waves Iri- 

 descence of Striated Surfaces The Colors 

 of Mother-of-pearl Transferred to Black 

 Sealing-wax. By interference in the earth's 

 atmosphere the light of a star, as shown 

 by Arago, is self-extinguished, the twinkling 

 of the star and the changes of color which 

 it undergoes being due to this cause. Look- 

 ing at such a star through an opera-glass, 

 and shaking the glass so as to cause the 

 image of the star to pass rapidly over the 

 retina, you produce a row of colored beads, 

 the spaces between which correspond to the 

 periods of extinction. Fine scratches drawn 

 upon glass or polished metal reflect the 

 waves of light from their sides ; and some, 

 being reflected from opposite sides of the 

 same scratch, interfere with and quench each 

 other. But the obliquity of reflection which 

 extinguishes the shorter waves does not ex- 



tinguish the longer ones, hence the phe- 

 nomena of color. These are called the colors 

 of striated surfaces. They are beautifully 

 illustrated by mother-of-pearl. This shell 

 is composed of exceedingly thin layers, 

 which, when cut across by the polishing 

 of the shell, expose their edges and furnish 

 the necessary small and regular grooves. 

 The most conclusive proof that the colors 

 are due to the mechanical state of the sur- 

 face is to be found in the fact, established 

 by Brewster, that by stamping the shell 

 carefully upon black sealing-wax we trans- 

 fer the grooves, and produce upon the wax 

 the colors of mother-of-pearl. TYNDALL Lec- 

 tures on Light, lect. 2, p. 92. (A., 1898.) 



3506. TYPE, COMMON, TRACED 

 THROUGH ALL VERTEBRATES Legs of 

 Whale and Boa-constrictor. The general 

 law to be learned from the series of skele- 

 tons in a natural-history museum is that 

 through order after order of fishes, reptiles, 

 birds, beasts, up to man himself, a common 

 type or pattern may be traced, belonging 

 to all animals which are vertebrate that 

 is, which have a back-bone. Limbs may 

 still be recognized, tho their shape and 

 service have changed, and tho they may even 

 have dwindled into remnants, as if left not 

 for use, but to keep up the old model. Thus, 

 altho a perch's skeleton differs so much 

 from a man's, its pectoral and ventral fins 

 still correspond to arms and legs. Snakes 

 are mostly limbless, yet there are forms 

 which connect them with the quadrupeds, 

 as, for instance, the boa-constrictor's skele- 

 ton shows a pair of rudimentary hind legs. 

 The Greenland whale has no visible hind 

 limbs, and its fore limbs are paddles or flip- 

 pers ; yet when dissected, the skeleton shows 

 not only remnants of what in man would 

 be the leg-bones, but the flipper actually 

 has within it the set of bones which belong 

 to the human arm and hand. It is popu- 

 larly considered that man is especially dis- 

 tinguished from the lower animals by not 

 having a tail; yet the tail is plainly to be 

 seen in the human skeleton, represented by 

 the last tapering vertebrse of the spine. 

 TYLOR Anthropology, ch. 2, p. 36. (A., 1899.) 



3507. UNANIMITY, CONVINCING 

 POWER OF Astronomy Trusted Because 

 Astronomers Agree. In order that this salu- 

 tary ascendency over opinion should be exer- 

 cised by the most eminent thinkers it is not 

 necessary that they should be associated and 

 organized. The ascendency will come of it- 

 self when the unanimity is attained, with- 

 out which it is neither desirable nor pos- 

 sible. It is because astronomers agree in 

 their teaching that astronomy is trusted, and 

 not because there is an Academy of Sciences 

 or a Royal Society issuing decrees or passing 

 resolutions. MILL Positive Philosophy of 

 Auguste Comte, p. 91. (H. H. & Co., 1887.) 



3508. Spectrum Anal- 

 ysis Universally Accepted as Trustworthy. 

 I do not know any more remarkable fact 



