x.] CONTORTIONS EXPLAINED. 137 



position of the line would be changed towards the region of 

 longer waves i.e. towards the red. 



Let us suppose such a change to be observed, say a change of 

 the F line, the normal wave-length of which is 0'00048606 from 

 that position to 0*00048716. Obviously the wave has been 

 lengthened by the recession of the source of light from the eye, 

 and the amount of recession, about thirty-nine miles a second, 

 is measured by the increased length of wave, the difference in 

 the wave-length bearing the same ratio to the total wave-length 

 as the difference in the velocity bears to the velocity of light. 1 



We can now interpret the meaning of the strange doubling of 

 the F line shown in Fig. 54. We there get, according to the 

 principle just laid down, an indication of the fact that the 

 hydrogen up to a certain height was very nearly at rest, and 

 that beyond, part of it was torn away, the line being deflected 

 towards the blue, indicating that it was approaching us. Now 

 the Fraunhofer lines in the diagram may be looked upon as so 

 many milestones which enable us to measure by the deflection 

 the number of miles traversed by the gas in one second; for 

 these deflections are nothing more nor less than alterations of 

 wave-length, and, thanks to Angstrom's map, we can measure 

 distances along the spectrum in TwoWo-zr mm., and we know 

 that an alteration of Tiruwoinj- mm - * n tne wave-length of the 

 F line towards the violet means a velocity of thirty-eight miles 

 a second towards the eye ; and that a similar alteration towards 

 the red means a similar velocity from the eye ; so that carrying 

 the part of the line which has the greatest deflection from the 

 normal down to the dots, we find that the velocity of the solar 

 wind under observation at that time was something like 114 

 miles per second. 



In the second figure this same prominence is seen a short 

 time afterwards. The tremendous rush of hydrogen has 

 descended somewhat nearer the sun, and bringing that in the 

 1 See Clerk-Maxwell, Phil. Trans. 1868, p. 532. 



