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THE POPULAR SCIENCE MONTHLY.— SUPPLEMENT. 



the Greenwich observers have achieved an un- 

 doubted success ; but unfortunately for them, 

 though fortunately for science, another observa- 

 tory, far smaller and of much less celebrity, has 

 at the critical moment achieved success still 

 more complete. 



The astronomers at our national observatory 

 have been able to recognize by the new method 

 the turning-motion of the sun upon his axis. 

 And here we have not, as in the case of Venus, 

 to recoi'd merely that the observers have seen 

 what they expected to see because of the known 

 motion of the sun. " Particular care was taken," 

 says Airy, "to avoid any bias from previous 

 knowledge of the direction in which a displace- 

 ment" (of the spectral lines) ''was to be expect- 

 ed," the side of the sun under observation not 

 being known by the observer until after the ob- 

 servation was completed. 



But Prof. Young, at Dartmouth College, Han- 

 over, New Hampshire, has done much more than 

 merely obtain evidence by the new method that the 

 sun is rotating as we already knew. He has suc- 

 ceeded so perfectly in mastering the instrumental 

 and observational difficulties, as absolutely to be 

 able to rely on his measurement (as distinguished 

 from the mere recognition) of the sun's motion of 

 rotation. The manner in which he has extended 

 the powers of ordinary spectroscopic analysis can- 

 not very readily be described in these pages, sim- 

 ply because the principles on which the extension 

 depends require for their complete description a 

 reference to mathematical considerations of some 

 complexity. Let it be simply noted that what is 

 called the diffractive spectrum, obtained by using 

 a finely-lined plate, results from the dispersive 

 action of such a plate, or grating, as it is techni- 

 cally called, and this dispersive power can be 

 readily combined with that of a spectroscope of 

 the ordinary kind. Now, Dr. Rutherford, of New 

 York, has succeeded in ruling so many thousand 

 lines on glass within the breadth of a single inch 

 as to produce a grating of high dispersive power. 

 Availing himself of this beautiful extension of 

 spectroscopic powers, Prof. Young has succeeded 

 in recognizing effects of much smaller motions 

 of recession and approach than had before been 

 observable by the new method. He has thus 

 been able to measure the rotation-rate of the 

 sun's equatorial regions. His result exceeds con- 

 siderably that inferred from the telescopic obser- 

 vation of the solar spots. For, whereas from 

 the motion of the spots a rotation-rate of about 

 one and a quarter mile per second has been cal- 

 culated for the sun's equator, Prof. Young ob- 



tains from his spectroscopic observations a rate 

 of rather more than one and two-fifths mile, or 

 about 300 yards per second more than the tele- 

 scopic rate. 



If Young had been measuring the motion of 

 the same matter which is observed with the tele- 

 scope, there could of course be no doubt that the 

 telescope was right, and the spectroscope wrong. 

 We might add a few yards per second for the 

 probably greater distance of the sun resulting 

 from recent transit observations. For, of course, 

 with an increase in our estimate of the sun's dis- 

 tance there comes an increase in our estimate of 

 the sun's dimensions, and of the velocity of the 

 rotational motion of his surface; but only about 

 twelve yards per second could be allowed on this 

 account, the rest would have to be regarded as 

 an error due to the difficulties involved in the 

 spectroscopic method. But in reality the tele- 

 scopist and the spectroscopist observe different 

 things in determining by their respective meth- 

 ods the sun's motion of rotation. The former 

 observes the motion of the spots belonging 

 to the sun's visible surface ; the latter observes 

 the motion of the glowing vapors outside that sur- 

 face, for it is from thes-e vapors, not from the 

 surface of the sun, that the dark lines of the 

 spectrum proceed. Now, so confident is Prof. 

 Young of the accuracy of his spectroscopic ob- 

 servations, that he is prepared to regard the 

 seeming difference of velocity between the at- 

 mosphere and surface of the sun as real. He be- 

 lieves that " the solar atmosphere really sweeps 

 forward over the underlying surface in the same 

 way that the equatorial regions outstrip the other 

 parts of the sun's surface." This inference, im- 

 portant and interesting in itself, is far more im- 

 portant in what it involves. For, if we can ac- 

 cept it, it follows that the spectroscopic method 

 of measuring the velocity of motions in the line 

 of sight is competent, under favorable conditions, 

 to obtain results accurate within a few hundred 

 yards per second, or ten or twelve miles per 

 minute. If this shall really prove to be true for 

 the method now, less than nine years after it was 

 first successfully applied, what may we not hope 

 from the method in future years ? Spectroscopic 

 analysis itself is in its infancy, and this method is 

 but a recent application of spectroscopy. A cen- 

 tury or so hence astronomers will smile (though 

 not disdainfully) at these feeble efforts, much as 

 we smile now in contemplating the puny tele- 

 scopes with which Galileo and his contemporaries 

 studied the star-depths. And we may well be- 

 lieve that largely as the knowledge gained by 





