SPECTROSCOPIC PROOF OF THE REPULSION BY THE 



SUX OF GASEOUS AlOLECULES IN THE TAIL 



OF HALLEY'S COMET. 



By PERCIVAL LOWELL. 

 {Read April Ji, 191 1.) 



1. The return of Halley's comet has been noteworthy chiefly for 

 the possibihty of employing upon it modern methods of instrumental 

 research. Since its last previous apparition have been devised those 

 two great engines of astronomic exploration, spectroscopy and celes- 

 tial photography. The former has afiforded us our first direct knowl- 

 edge of the substances composing comets, while the latter has given 

 us a means of easy and rapid registration of the visitant's appear- 

 ance. This is especially valuable in the case of a body as vast and 

 vague as a comet, free-hand drawing of which is peculiarl}- liable 

 to distortion. 



During the last return of Halley's comet that body was sub- 

 jected at Flagstaif to investigation b\' both instruments simulta- 

 neously. One result of this was the detection that gaseous mole- 

 cules — in contradistinction to minute solid particles merely — are 

 directly repelled b}- a force emanating from the sun, presumably 

 the pressure of light. Previously this had been held impossible. 

 Schwarzschild had, as he thought, demonstrated mathematically in 

 an able paper that molecules of gas were too small to be thus aiTected 

 by the forces concerned and Arrhenius had adopted his deduction 

 and published it as a fact in his " Worlds in the Making." That 

 the bodies themselves would so soon refute this would not have been 

 deemed probable and invests the detection with the more immediate 

 interest. Incidentally we may remark that vSchwarzschild had since 

 given up his original opinion. 



2. That the tail of a comet is due to repellant force exerted by 

 the sun is api)arent from the direction the tail takes. For that direc- 



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