DRIFTING LIGHT-WAVES. 



323 



the new method is trustworthy at all, to recog- 

 nize this marked difference between the state of 

 the sun's eastern and western edges ; found on 

 trial that he could not do so; and accordingly 

 expressed his opinion that the new method is not 

 trustworthy, and that the arguments urged in its 

 favor are invalid. 



The weak point in his reasoning resided in 

 the circumstance that the solar equator is only 

 moving at the rate of about one and a quarter 

 mile per second, so that instead of a difference 

 of forty miles per second between the two edges, 

 which should be appreciable, the actual differ- 

 ence (that is, the sum of the two equal motions 

 in opposite directions) amounts only to two and 

 a half miles per second, which certainly Secchi 

 could not hope to recognize with the spectro- 

 scopic power at his disposal. Nevertheless, when 

 the error in his reasoning was pointed out, though 

 he admitted that error, he maintained the justice 

 of his conclusion ; just as Cassini, having mis- 

 takenly reasoned that the degrees of latitude 

 should diminish toward the pole instead of in- 

 creasing, and having next mistakenly found, as 

 he supposed, that they do diminish, acknowledged 

 the error of his reasoning, but insisted on the 

 validity of his observations ; maintaining thence- 

 forth, as all the world knows, that the earth is 

 extended instead of flattened at the poles ! 



Huggins tried to recognize by the new meth- 

 od the effects of the sun's rotation, using a much 

 more powerful spectroscope than Secchi had em- 

 ployed. The history of the particular spectro- 

 scope he employed is in one respect specially 

 interesting to myself, as the extension of spec- 

 troscopic power was of my own devising before 

 I had ever used or even seen a powerful spectro- 

 scope. The reader is aware that spectroscopes 

 derive their light-sifting power from the prisms 

 forming them. The number of prisms was gradu- 

 ally increased, from Newton's single prism to 

 Fraunhofer's pair, Kirchhoffs battery of four, 

 till six were used, which bent the light round as 

 far as it would go. Then the idea occurred of 

 carrying the light to a higher level (by reflec- 

 tions) and sending it back through the same 

 battery of prisms, doubling the dispersion. Such 

 a battery, if of six prisms, would spread the 

 spectral colors twice as widely apart as six used 

 in the ordinary way, and would thus have a dis- 

 persive power of twelve prisms. It occurred to 

 me that after taking the rays through sis prisms, 

 arranged in a curve like the letter C, an inter- 

 mediate four-cornered prism of a particular 

 shape (which I determined) might be made to 



send the rays into another battery of six prisms, 

 the entire set forming a double curve like the 

 letter S, the rays being then carried to a higher 

 level and back through the double battery. In 

 this way a dispersive power of nineteen prisms 

 could be secured. My friend Mr. Browning, the 

 eminent optician, made a double battery of this 

 kind, 1 which was purchased by Mr. W. Spottis- 

 woode, and by him lent to Mr. Huggins for the 

 express purpose of dealing with the task Secchi 

 had set spectroscopists. It did not, however, 

 afford the required evidence. Huggins consid- 

 ered the displacement of dark lines due to the 

 sun's rotation to be recognizable, but so barely 

 that he could not speak confidently on the point. 



There for a while the matter rested. Vogel 

 made observations confirming Huggins's results 

 relative to stellar motions ; but Vogel's instru- 

 mental means were not sufficiently powerful to 

 render his results of much weight. 



But recently two well-directed attacks have 

 been made upon this problem, one in England, 

 the other in America, and in both cases with 

 success. Rather, perhaps, seeing that the meth- 

 od had been attacked and was supposed to re- 

 quire defense, we may say that two well-directed 

 assaults have been made upon the attacking 

 party, which has been completely routed. 



Arrangements were made not very long ago, 

 by which the astronomical work of Greenwich 

 Observatory, for a long time directed almost ex- 

 clusively to time observations, should include the 

 study of the sun, stars, planets, and so forth. 

 Among other work which was considered suited 

 to the national observatory was the application 

 of spectroscopic analysis to determine motions 

 of recession and approach among the celestial 

 bodies. Some of these observations, by-the-way, 

 were made, we are told, " to test the truth of 

 Doppler's principle," though it seems difficult to 

 suppose for an instant that mathematicians so 

 skillful as the chief of the observatory and some 

 of his assistants could entertain any doubt on 

 that point. Probably it was intended by the 

 words just quoted to imply simply that some of 

 the observations were made for the purpose of 



i I have omitted all reference to details; hut in reality 

 the douhle battery was automatic, the motion of the ob- 

 serving telescope, as different colors of the spectrum were 

 brought into view, setting all the prisms of the double 

 battery into that precise position which causes them to 

 show best each particular part of the spectrum thus 

 brought into view. It is rather singular that the first view 

 I ever had of the solar prominences was obtained (at Dr. 

 Huggins's observatory) with this instrument of my own 

 invention, which also was the first powerful spectroscope 

 I had ever used or even seen. 



