28 BEPORT— 1881. 



only, indicating that ' in place of an incandescent Bolid or liquid body 

 we must probably regard these objects, or at least their photo-surfaces, 

 as enormous masses of luminous gas or vapour. For it is from matter 

 in a gaseous state only that such light as that of the nebulae is known 

 to be emitted.' So far as observation has yet gone, nebulte may be 

 divided into two classes : some giving a continuous spectrum, others one 

 consisting of bright lines. These latter all appear to give essentially the 

 same spectrum, consisting of a few bright lines. Two of them, in Mr. 

 Huggins' opinion, indicate the presence of hydrogen : one of them agrees 

 in position with a line characteristic of nitrogen. 



But spectrum analysis has even more than this to tell us. The old 

 methods of observation could determine the movements of the stars so 

 far only as they were transverse to us ; they afforded no means of 

 measuring motion either directly towards or away from us. Now 

 Doppler suggested in 1841 that the colors of the stars would assist us 

 in tins respect, because they would be affected by their motion to and 

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

 proaches or recedes from us. Everyone 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 whicb reach the ear 

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

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

 would be affected by such a movement ; but Doppler's method was prac- 

 tically inapplicable, not only because the amount of effect on the color 

 would be hardly sensible, but also for other reasons ; indeed, as we did 

 not know the true color of the stars, we had no datum line by which to 

 measure. 



A cliange of refrangibility of light, bow'ever, does occur in conse- 

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

 scope to solve the problem. He took in the first place the spectroscope 

 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 contrary, if Sirius were ap- 

 proaching or receding from us, this line would be slightly shifted either 

 towards the blue or red end of the spectrum. He found that the line 

 had moved very slightly towards the red, indicating 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 Astronomer Royal, in succession to Sir G. Airy, who 

 has long occupied the post with so much honour to himself and advantage 

 to science, 



