February 6, 1896] 



NA rURE 



32 



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 

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

 Dr. Frankland and myself found that there was very 

 considerable justification for this view, because a simple 

 calculation showed that the thickness of the solar atmo- 

 sphere, which was producing that yellow line under the 

 conditions which enabled us to see it in our instruments 

 by looking along the edge of the sun, was something 

 like 2oo,ocx) miles. 



Fig. 6. — Changes of wave-Iengtn of the F hydrogen line when a solar cyclone is observed. 

 K, 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 

 frcm 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 blue line, 

 for instance, might scmetimes be slightly on one side, 

 and scmetimes on the other of its normal position. 

 Further work showed that in these so-called "changes of 



my first observations, several new lines about which 

 nothing was known were thus observed. The place of 

 this orange line I determined on October 20, 1868. Among 

 many other lines behaving like it, two at wave-Iengths^ 

 4923 and 5017 were discovered in June 1869, and after- 

 wards another at 6677, while Prof. Young noted another 

 in September 1869, at 4471. He wrote : 



" I desire to call special attention to 2581*5 [Kirchhoff's 

 scale], the only one of my list, by the way, which is 

 not given on Mr. Lockyer's. This line, which was 

 conspicuous at the Eclipse of 1869, seems to be always 

 ■present in the spectrum of the chromosphere. . . . 

 It has no corresponding dark line in the 

 ordinary solar spectrum, and not improbably 

 may be due to the same substance that pro- 

 duces D'^" 



This same line was noted also by Loren- 

 zoni, and named y". 



Then with regard to solar disturbances. 

 Let me refer in detail to a diagram indicating 

 some results arrived at by the Italian ob- 

 servers. We are dealing with the spectro- 

 scopic record of two slight disturbances in a 

 particular part of the sun's atmosphere. The 

 spectroscope told us that in that region there 

 was a quantity of the vapour of magnesium 

 which was collected in that place. Then we 

 find that another substance, about which we 

 again know nothing whatever, is also visible 

 in that region, and then we get the further 

 fact that m those particular disturbances we 

 get four other spectral lines indicated as- 

 being disturbed, and of those four lines we only know 

 about one. 



In that way it very soon became perfectly clear to those 

 who were working at the sun, that in all these disturb- 

 ances, or at all events in most of them, we were dealing 

 to a large extent with lines not seen in cur laboratories 

 when dealing with terrestrial substances ; this work went 

 on till ultimately, thanks to the labours of Prof. Young 

 in America, we had a considerable list of lines coming 



wave-length" we had a precious means of determining | from known and unknown substances which had been ob- 

 the rate of movement of the gases and vapours in the | 

 solar atmosphere. 



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

 shown in the spectroscope. The lines are contorted 

 in both directions, and sometimes to a very considerable 

 extent, indicating wind-movements on the sun, reaching, 

 and sometimes exceeding, ico miles a second! 



Well, then, you see we had here a means of deter- 

 mining whether the yellow line was produced by the 

 same gases which gave the red and blue lines, because 

 if so, when we got any alteration in the position of the 

 red and blue lines, which always worked together, we 

 should get an equivalent alteration in the position of the 

 yellow one. 



I found that the yellow line behaved quite differently \ 

 from either the red or the blue line ; so then we knew that , 

 we were not dealing with hydrogen ; hence we had to ' 

 do with an element which we could not get in our \ 

 laboratories, and therefore I took upon myself the re- j 

 sponsibility of coining the word helium, in the first 

 instance for laboratory use. 



This kind of work went on for a considerable time, 

 and what one found was, that very often in solar dis- 

 turbances we certainly were dealing with some of the 

 lines of substances with which we are familiar on this 

 earth ; but at the same time it was very remarkable 

 that when the records came to be examined, as they 

 ultimately were with infinite care and skill, it was found 

 that not only did we get this line in the yellow indicating 

 an unknown element associated with substances very 

 well known, like magnesium, but that there were many 

 other unknown lines as well. Within a few months of 



NO. 137 1, VOL. 53] 



Fig. 7.— Tacchini's observations of two slight solar disturbances showing the 

 height to which the layers of the different gases extend. Magnesiuni 

 vapour is highest of all, and is furthest extended ; next comes a gas of 

 still unknown origin, indicated by a line at 1474 of KirchhofTs scale, and 

 so on. 



served under these conditions in the solar chromosphere 

 and Prof. Young was enabled to indicate the relative 

 number of times these lines were visible. For instance, 

 the lines which are most frequently seen under these 

 conditions he indicated as represented by the number 

 100, and of course the line which was least frequently 



