THE GROWTH OF OUR KNOWLEDGE OF HELIUM. 253 



or observing the spectrum of a spark in a stream of coal- 

 gas. 



Now at the first blush it looked very much as if this 

 line was really due to the same element which produced 

 the others at C and F, and it was imagined that the reason 

 we did not see it in our laboratories was because it was a 

 line which required a very considerable thickness of hydro- 

 gen to render it visible. That was the first idea, and Dr. 

 Frankland and myself found that there was very consider- 

 able justification for this view, because a simple calculation 

 showed that the thickness of the solar atmosphere, which 

 was producing that orange line under the conditions which 

 enabled us to see it in our instruments by looking along the 

 edge of the sun, was something like 200,000 miles. 



ABC 

 Fig. 6. — Changes of wave-length of the F hydrogen line when a solar 

 cyclone is observed. A, the change towards the red indicates the 

 retreating side of cyclone. C, the change towards the blue indicates 

 the advancing side. B, the whole cyclone is included in the width of 

 the slit, and both changes of wave-length are visible. 



Hence, in order to get a final decision on this point, 

 there was nothing for it but to tackle the question from a 

 perfectly different point of view, and the different point of 

 view was this. The work had not gone on very long 

 before one found minute alterations in the positions of these 

 lines in the spectrum ; the orange line, for instance, might 

 sometimes be slightly on one side, and sometimes on the 

 other of its normal position. Further work showed that in 

 these so-called " changes of wave-length " we had a precious 

 means of determining the rate of movement of the gases 

 and vapours in the solar atmosphere. 



Fig. 6 indicates how these changes of wave-lengths are 



