Exactness 



SCIENTIFIC SIDE-LIGHTS 



230 



hand may be so magnified by this method as 

 to cause the index-beam to move through 20 

 or 30 feet. The lengthening of a bar of iron 

 when it is magnetized may be also thus 

 demonstrated. TYNDALL Lectures on Light, 

 lect. 1, p. 12. (A., 1898.) 



1119. 



Lines of the Spec- 



trum Made to Tell Their Story Minerals in 

 the Sun Helium Found on Earth. It was 

 already known that the various chemical 

 elements, when heated to incandescence, pro- 

 duce spectra consisting of a group of colored 

 bands, and it had been noticed that some of 

 these bands, as the yellow band of sodium, 

 corresponded in position with certain black 

 lines in the solar spectrum. KirchofFs dis- 

 covery consisted in showing that, when the 

 light from an incandescent body passes 

 through the same substance in a state of 

 vapor, much of it is absorbed, and the col- 

 ored bands become replaced by black lines. 

 The black lines in the solar spectrum are 

 due, on this theory, to the light from the in- 

 candescent body of the sun being partially 

 absorbed in passing through the vapors 

 which surround it. This theory led to a 

 careful examination of the spectra of all 

 the known elements, and 'on comparing them 

 with the solar spectrum it was found that 

 in many cases the colored bands of the ele- 

 ments corresponded exactly in position with 

 certain groups of black lines in the solar 

 spectrum. Thus hydrogen, sodium, iron, 

 magnesium, copper, zinc, calcium, and many 

 other elements have been proved to exist in 

 the sun. Some outstanding solar lines, 

 which did not correspond to any known ter- 

 restrial element, were supposed to indicate 

 an element peculiar to the sun, which was 

 therefore named helium. Quite recently this 

 element has been discovered in a rare min- 

 eral, and its colored spectrum is found to 

 correspond exactly to the dark lines in the 

 solar spectrum on which it was founded, 

 thus adding a final proof of the correctness 

 of the theory, and affording a striking ex- 

 ample of its value as an instrument of re- 

 search. WALLACE The Wonderful Century, 

 ch. 6, p. 43. (D. M. &Co., 1899.) 



11 2O. Measurement of 



Vision Light-intensities Measurable by 

 Galvanic Current. It only remains for us 

 in this case to determine the least light- 

 intensity, which is in absolute darkness just 

 noticeably brighter than the black of the 

 field of vision. We can most easily obtain 

 very weak light-intensities of this kind by 

 passing a constant current through a metal 

 wire. As we increase the intensity of the 

 current, the wire becomes hotter and hotter, 

 till at a definite temperature it begins to be 

 luminous. And since we can graduate the 

 strength of a galvanic current at our pleas- 

 ure, the intensity at which the luminosity 

 of the wire becomes just noticeable can be 

 readily determined. We have then only to 

 compare its objective value with that of 

 other known light-intensities. It has been 



found in this way that the just noticeable 

 intensity of light is approximately -$^-3 

 of the light of the full moon reflected from 

 white paper. WUNDT Psychology, lect. 4, p. 

 54. (Son. & Co., 1896.) 



1121. 



Measuring the Sal" 



mon's Leap. The distances up rivers to 

 which salmon will swim in the spawning 

 season is no less surprising than the energy 

 with which they perform the feat, and the 

 determination with which they overcome all 

 obstacles. They reach Bohemia by the Elbe 

 and Switzerland by the Rhine. On encoun- 

 tering a waterfall they display astonishing 

 agility and perseverance in surmounting the 

 obstacle. This fact, of course, is well known 

 to all salmon-fishers; but the actual ver- 

 tical height to which a well-grown salmon is 

 able to leap has only recently (1886) been 

 made the subject of exact measurement. By 

 means of upright posts fixed upon the banks 

 of a stream on either side of a waterfall in 

 Norway, Prof essor Landmark has determined 

 that this fish is able to rise through the air, 

 by a single spring, a vertical distance of 

 sixteen feet. The salmon, therefore, may be 

 said to have no competitor in its perform- 

 ance of the high jump, unless it be the kan- 

 garoo, as to whose powers in this respect I 

 have not been able to find trustworthy in- 

 formation. ROMANES Animal Intelligence f 

 ch. 8, p. 249. (A., 1899.) 



1122. 



Minute Displace- 



ment of Sirius Still More Minute Correc- 

 tion. The annual displacement of Sirius 

 may be thus illustrated: On a clear moon- 

 light night let the reader notice the appar- 

 ent diameter of the moon. Next let him try 

 to conceive that diameter divided into about 

 3,800 equal parts. Then the greatest displace- 

 ment of Sirius is equal to one of those mi- 

 nute portions. Sirius in fact appears to 

 circle round a minute oval path on the 

 heavens, having for its longest diameter a 

 space equal to about the 3,800th part of the 

 moon's apparent diameter. Now, the error 

 of the earlier estimate ( supposing that esti- 

 mate erroneous ) consisted in setting the dis- 

 placement of Sirius at about the 6,300th 

 part of the moon's diameter the difference 

 between the two estimates corresponding to 

 about the 9,500th part of the moon's appar- 

 ent diameter. If the reader will but con- 

 ceive the moon's apparent diameter divided 

 into about 100 parts, and one of these parts 

 again into 100 parts, he will be able to form 

 an idea of the exceeding minuteness of the 

 quantity by which astronomers suppose that 

 their first estimate was erroneous. PROC- 

 TOR Our Place among Infinities, p. 166. (L. 

 G. & Co., 1897.) 



1123. 



Newton Calculates 



the Depth of Fine Film of Air. Newton 

 compared the tints obtained in this way [a 

 plate of glass with a plane surface being 

 laid on a plano-convex glass lens of very 

 feeble curvature] with the tints of his soap- 



