February 13, 1896] 



NATURE 



343 



build up its light, and whether the light-notes from 

 cver>' part of the nebula are absolutely the same, or 

 whether there are differences of chemical constitution in 

 different parts. In this way it was found many years ago 

 that both in the light of stars and of nebuhe we get very 

 considerable varieties of spectra. Let me prove this 

 point by referring to some recent observations. 



The original negative of the nebula of Orion, taken at 

 my Observatory at Westgate-on-Sea in 1890, contains 

 fifty-six lines, and of course by determining, as we have 

 been able to do, the wave-lengths — the positions of these 

 lines in the spectrum — we can determine the exact light- 

 notes represented, and therefore the substances which 



NO. 1372, VOL. 53] 



produce them. Now I have to tell you that in this 

 spectrum of the nebula of Orion we get lines of unknown 

 origin exactly coinciding with those unknown lines which 

 I have already referred to as having been seen in the sun's 

 atmosphere. Mark it well, that some of the unknown 

 lines in that atmosphere, those that we have not been 

 able to see in our laboratories, are identical in position 

 with some of the unknown lines in the nebula of Orion, 

 the line D^ being one case in point. 



Then, I have next to say that in the spectra of many 

 stars we have these same lines, which we have so far 

 classified as unknown for the reason above stated, that in 

 our laboratories we have not been able to get any lines 

 which correspond with them. 



I call attention to the spectra of some stars which have 

 been carefully photographed and measured. You will see 

 that the progress of this branch of science lately has 

 been so considerable that any statement made with 

 regard to the positions of lines, and therefore the 

 chemical origins of them, may be made with a consider- 

 able amount of certainty as depending upon very accurate 

 work. 



It forms no part of my present purpose to indicate the 

 various classes in which the stars have been classified 

 by different observers according to their spectra, but 

 some of the more salient differences must be pointed out. 

 Thus we have stars with many lines in their spectra, 

 others with comparatively few. I will take the many- 

 lined stars first. 



The diagram (Fig. 9) represents the spectrum of Arc- 

 turus, a star the spectrum of which closely resembles that 

 of the sun. In o Cygni we have another star with many 

 lines, but here we note, when we leave the hydrogen on 

 one side and deal with the other stronger lines, that 

 there is little relation between the solar spectrum ^nd 

 these lines. 



I next come to the stars with few lines : these are well 

 represented by many of the chief stars in the Constellation 

 of Orion. Bellatrix is given as an example (Fig. 10). 



Some astronomers hold the view that all the stars were 

 hotter when they were first formed, than they ever have 

 been afterwards. I have attempted to prove not only that 

 this view is unphilosophic, but, further, that the spectro- 

 scopic facts indicate that we can arrange the stars along 

 one line, provided the hottest stars are supposed to occupy 

 the middle of it. In other words, that stars begin cool, 

 get hotter, and then cool down finally. 



In this way we are enabled to trace the chemical 

 changes from one star to another, and that has been done 

 with an expenditure of considerable labour. 



Chapter IV. 



Now I go to another chapter of my story. I ask 

 you to accompany me in your mind's eye to another 

 eclipse. The importance of eclipses in the abstract I 

 think you will grant, from some of the results which 1 

 have already referred to. A method which was first em- 

 ployed by Respighi and myself during the eclipse of 

 1 87 1, was employed on a large scale and with great effect 

 during the eclipse of 1893. The light proceeding from 

 the luminous ring round the dark moon was made to give 

 us a series of rings, representing each bright line seen by 

 the ordinary method, on a photographic plate. The ob- 

 servers this time were stationed in West Africa and in 

 Brazil. The African station was up one of the rivers, not 

 very far away from the town of Bathurst. 



As a matter of fact, the very same instrument which was 

 previously referred to as used for obtaining photographs 

 of the stars was sent here, in order that photographs of the 

 eclipse of the sun might be taken on exactly the same 

 scale as the photographs of the stars had been. We next 

 come to the Brazilian station. Here again the instruments 

 were somewhat similar. I have another view of the 



