448 THE POPULAR SCIENCE MONTHLY. 



a velocity comparable with that of the water-waves, or no change 

 will be observed. The trains of the second illustration must have 

 a velocity comparable with that of the aerial waves producing sound, 

 or no change of tone will be j^roduced. And in like manner a star or 

 other celestial body must be approaching us, or receding from us, 

 with a velocity comparable with that of the ethereal waves producing 

 light, or no change of color will be produced, the color of light cor- 

 responding with the tone of sound. Unfortunately for this purpose, 

 though most fortunately in other respects, light travels at so enor- 

 mous a rate that even the swiftest motions of the heavenly bodies 

 seem rest by comparison. What, for instance, is the rush of even 

 Newton's comet past its point of nearest approach to the sun, though 

 at the rate of more than 300 miles per second, to the flight of light 

 over nearly 200,000 miles in the same time? "Very much as the 

 movement of a person taking only six steps a minute, each less than 

 half a yard long, to the rush of the swiftest express=train. Yet as- 

 tronomers have undertaken to measure the approach and recession of 

 stars, moving in some cases with less than a tenth part of that 

 comet's motion, and whose velocity, therefore, sinks into still more 

 utter insignificance by comparison with that of light. 



Secchi claims (but not justly) to have first invented and applied 

 the method used for this purpose, which consists in noting whether 

 some known line of the spectrum of a heavenly body changes in 

 position either by moving toward the violet end of the spectrum, 

 which would imply approach, or by moving toward the red end, 

 which would imply recession. Of course, the method is exceedingly 

 delicate and difficult, involving a number of details which would be 

 quite unsuited to these pages ; but that, in principle, is its nature. 

 Secchi tried the method, and failed to get any results from it, an- 

 nouncing his unsuccessful attempt in March, 1868. " Then," he says, 

 "Mr. Huggins retried (reprit) the method, announcing in April, 1868, 

 the discovery that Sirius is receding at the rate of twenty miles per 

 second." Secchi should know well, however, that our great spectro- 

 scopist did not achieve this success in the few weeks between Sec- 

 chi's announcement of failure and Huggins's announcement of suc- 

 cess. Months had elapsed, during which Huggins had been strug- 

 gling with this difficult problem. Tf the enunciation of the method 

 gave claim to the credit of its successful application, I myself could 

 advance a stronger claim than Secchi's, for in an essay in Fraser' ) s 

 Magazine for January, 1868, I definitely indicated the nature and 

 value of the method. But I would rather refer to the circumstance 

 as enabling me to support Huggins's assertion that he was observing 

 by this method for months before Secchi announced his own failure, 

 for immediately on the appearance of my essay I received a letter 

 from Dr. Huggins, mentioning (in confidence, until his paper should 

 be published) that he had been for some time striving for success 



