Mr. J. N. Lockyer on Recent Discoveries in Solar Physics. 153 



So that, generally, representing the interval between each soldier 

 by an elastic cord, if the barrack and the eye approach each other by 

 the motion of either, the cord will contract ; in the case of recession, 

 the cord will stretch. 



Now let the barrack represent the hydrogen on the sun perpetu- 

 ally paying out waves of light, and let the elastic cord represent one 

 of these waves ; its length will be changed if the hydrogen and the 

 eye approach each other by the motion of either. 



Particular wave-lengths with the normal velocity of light are re- 

 presented to us by different colours. 



The long waves are red. 



The short waves are violet. 



Now let us fix our attention on the green wave, the refrangibility 

 of which is indicated by the F line of hydrogen. If any change of 

 wave-length is observed in this line, and not in the adjacent ones, it is 

 clear that it is not to the motion of the earth or sun, but to that of 

 the hydrogen itself and alone that the change must be ascribed. 



If the hydrogen on the sun is approaching us, the waves will be 

 crushed together ; they will therefore be shortened, and the light will 

 incline towards the violet — that is, towards the light with the short- 

 est waves ; and if the waves are shortened only by the yuiToVuTTO 

 of a millimetre we can detect the motion. 



If the hydrogen on the sun is receding from us, the waves will be 

 drawn out ; they will therefore be longer, and the green ray will in- 

 cline towards the red. 



I must next point out that there are two different circumstances 

 under which the hydrogen may approach or recede from the eye. 



I have here a globe, which we will take as representing the sun. 

 Fix your attention on the centre of this globe : it is evident that an 

 uprush or a downrush is necessary to cause any alteration of wave- 

 length. A cyclone or lateral movement of any kind is powerless ; 

 there will be no motion to or from the eye, but only at right angles 

 to the line of sight. 



Next fix your attention on the edge of the globe — the limb, in 

 astronomical language ; here it is evident that an upward or down- 

 ward movement is as powerless to alter the wave-length as a la- 

 teral movement was in the other case, but that, should any lateral or 

 cyclonic movement occur here of sufficient velocity, it might be de- 

 tected. 



So that we have the centre of the disk for studying upward and 

 downward movements, and the limb for studying lateral or cyclonic 

 movements, if they exist. 



If the hydrogen-lines were invariably observed to broaden out on 

 both sides, the idea of movement would require to be received with 

 great caution ; we might be in presence of phenomena clue to greater 

 pressure, either when the lines observed are bright or black upon the 

 sun ; but when they widen out, sometimes on one side, sometimes 

 on the other, and sometimes on both, this explanation appears to be 

 untenable, as Dr. Frankland and myself in our researches at the Col- 

 lege of Chemistry have never failed to observe a widening out, equally 



