196 THE POPULAR SCIENCE MONTHLY. 



from the earth, just as a steam-whistle is raised or lowered as it ap- 

 proaches or recedes from us. Every one has observed that if a train 

 whistles as it passes us, the sound appears to alter at the moment the 

 engine goes by. This arises, of course, not from any change in the 

 whistle itself, but because the number of vibrations which reach the 

 ear in a given time are increased by the speed of the train as it 

 approaches, and diminished as it recedes. So, like the sound, the color 

 would be affected by such a movement ; but DOppler's method was 

 practically inapplicable, because the amount of effect on the color 

 would be utterly insensible ; and even if it were otherwise, the method 

 could not be applied, because, as we did not know the true color of 

 the stars, we have no datum-line by which to measure. 



A change of refrangibility of light, however, does occur in conse- 

 quence of relative motion, and Huggins successfully applied the spec- 

 troscope to solve the problem. He took in the first place the spectro- 

 scope of Sirius, and chose a line known as F, which is due to hydrogen. 

 Now, if Sirius was motionless, or rather if it retained a constant 

 distance from the earth, the line F would occupy exactly the same 

 position in the spectrum of Sirius as in that of the sun. On the con- 

 trary, if Sirius were approaching or receding from us, this line would 

 be slightly shifted either toward the blue or red end of the spectrum. 

 He found that the line had moved very slightly toward the red, indi- 

 cating that the distance between us and Sirius is increasing at the rate 

 of about twenty miles a second. So also Betelgeux, Rigel, Castor, 

 and Regulus are increasing their distance ; while, on the contrary, 

 that of others, as for instance of Vega, Arcturus, and Pollux, is 

 diminishing. The results obtained by Huggins on about twenty stars 

 have since been confirmed and extended by Mr. Christie, now As- 

 tronomer-Royal in succession to Sir G. Airy, who has long occupied 

 the post with so much honor to himself and advantage to science. 



To examine the spectrum of a shooting-star would seem even more 

 difficult ; yet Alexander Herschel has succeeded in doing so, and finds 

 that their nuclei are incandescent solid bodies ; he has recognized the 

 lines of potassium, sodium, lithium, and other substances, and con- 

 siders that the shooting - stars are bodies similar in character and 

 composition to the stony masses which sometimes reach the earth as 

 aerolites. 



Some light has also been thrown upon those mysterious visitants, 

 the comets. The researches of Professor Newton on the periods of 

 meteoroids led to the remarkable discovery by Schiaparelli of the iden- 

 tity of the orbits of some meteor-swarms with those of some comets. 

 The similarity of orbits is too striking to be the result of chance, and 

 shows a true cosmical relation between the bodies. Comets, in fact, 

 are in some cases at any rate groups of meteoric stones. From the 

 spectra of the small comets of 18G6 and 1868, Huggins showed that 

 part of their light is emitted by themselves, and reveals the presence 



