DEIFTIXG LIGHT- WA YES. 



321 



of those numbers which reason owns, while imagi- 

 nation ceases to comprehend them ; hut it is also 

 true that the swiftness with which certain individ- 

 uals of the double stars sweep past their perihelia, 

 or rather their periasters, is amazing; and, in this 

 matter of colors, it must be recollected that the 

 question solely regards the difference between the 

 velocity of the waves constituent of colors, at those 

 different stellar positions. Still it is a bold— even a 

 magnificent idea ; and if it can be reconciled with 

 the permanent colors of the multitude of stars sur- 

 rounding us— stars which too are moving in great 

 orbits with immense velocities— it may be hailed 

 almost as a positive discovery. It must obtain 

 confirmation, or otherwise, so soon as we can com- 

 pare with certainty the observed colorific changes 

 of separate systems with the known fluctuations 

 of their orbital motions." 



That was written a quarter of a century ago, 

 when spectroscopic analysis, as we now know it, 

 had no existence. Accordingly, while the fatal 

 objection to Doppler's original theory is over- 

 looked on the one hand, the means of applying the 

 principle underlying the theory, in a much more 

 exact manner than Doppler could have hoped 

 for, is overlooked on the other. Both points are 

 noted in the article above referred to, in the same 

 paragraph. " We may dismiss," I there stated, 

 " the theory started some years ago by the French 

 astronomer, M. Doppler." But, I presently added, 

 " It is quite clear that the effects of a motion 

 rapid enough to produce such a change" (i. e., a 

 change of tint in a pure color) " would shift the 

 position of the whole spectrum — and this change 

 would be readily detected by a reference to the 

 spectral lines." This is true, even to the word 

 "readily." Velocities •which would produce an 

 appreciable change of tint would produce "read- 

 ily" detectible changes in the position of the 

 spectral lines ; the velocities actually existing 

 among the star-motions would produce changes 

 in the position of these lines detectible only with 

 extreme difficult}', or perhaps in the majority of 

 instances not detectible at all. 



It has been in this way that the spectroscopic 

 method has actually been applied. 



It is easy to perceive the essential difference 

 between this way of applying the method and 

 that depending on the attempted recognition of 

 changes of color. A dark line in the spectrum 

 marks in reality the place of a missing tint. The 

 tints next to it on either side are present, but the 

 tint between them is wanting They are changed 

 in color — very slightly, in fact quite inapprecia- 

 bly — by motion of recession or approach, or, in 

 other words, they are shifted in position along 

 21 



the spectrum, toward the red end for recession, 

 toward the violet end for approach ; and, of 

 course, the dark space between is shifted along 

 with them. One may say that the missing tint is 

 changed. For in reality that is precisely what 

 would happen. If the light of a star at rest gave 

 every tint of the spectrum, for instance, except 

 mid-green alone, and that star approached or 

 receded so swiftly that its motion would change 

 pure green light to pure yellow in one case, or 

 pure blue in the other, then the effect on the spec- 

 trum of such a star would be to throw the dark 

 line from the middle of the green part of the 

 spectrum to the middle of the yellow part in one 

 case, or to the middle of the blue part in the 

 other. The dark line would be quite notably 

 shifted in either case. With the actual stellar 

 motions, though all the lines are more or less 

 shifted, the displacement is always exceedingly 

 minute, and it becomes a task of extreme diffi- 

 culty to recognize, and still more to measure, such 

 displacement. 



When I first indicated publicly (January, 

 ISfiS) the way in which Doppler's principle could 

 alone be applied, two physicists, Huggins in Eng- 

 land and Secchi in Italy, were actually endeavor- 

 ing, with the excellent spectroscopes in their pos- 

 session, to apply this method. In March, 1868, 

 Secchi gave up the effort as useless, publicly an- 

 nouncing the plan on which he had proceeded, and 

 his failure to obtain any results except negative 

 ones. A month later Huggins also publicly an- 

 nounced the plan on which he had been working, 

 but was also able to state that in one case, that 

 of the bright star Sirius, he had succeeded in 

 measuring a motion in the line of sight, having 

 discovered that Sirius was receding from the earth 

 at the rate of 41.4 miles per second. I say was 

 receding, because a part of the recession at the 

 time of observation was due to the earth's orbi- 

 tal motion around the sun. I had, at his request, 

 supplied Huggins with the formula for calculat- 

 ing the correction due to this cause, and, apply- 

 ing it, he found that Sirius is receding from the 

 sun at the rate of about 29^- miles per second, 

 or some 930,000,000 miles per annum. 



I am not here specially concerned to consider 

 the actual results of the application of this method 

 since the time of Huggins's first success ; but the 

 next chapter of the history of the method is one 

 so interesting to myself personally that I feel 

 tempted briefly to refer to details. So soon as I 

 had heard of Huggins's success with Sirius, and 

 that an instrument was being prepared for him 

 wherewith he might hope to extend the method 



