WHAT THE SUN IS MADE OF. 



317 



oar laboratories is equivalented by the intensity 

 of the so-called iron-lines which we assume we 

 observe in the spectrum of the sun. 



That is the great argument, in fact, for the 

 existence of iron in the sun. 



But, when we leave the iron group of metals, 

 we find others in which this coincidence, this 

 great similarity of intensity from one end of the 

 spectrum to the other, is considerably changed. 

 We get in the case of calcium very thick lines 

 corresponding with very thin lines in the sun, 

 and we get thin lines of calcium corresponding 

 with very thick lines in the sun. In fact, the two 

 thickest lines which have already been mapped in 

 the spectrum of the sun are lines due to calcium. 

 If we photograph the spectrum of calcium with a 

 very weak arc in an electric lamp, the lines most 

 obvious in the spectrum of the sun would scarce- 

 ly be visible at all on the photographic plate. If, 

 however, we pass from the tension of the arc to 

 the tension which is obtainable with the use of a 

 very large coil, then we can exactly equivalent 

 the spectrum which we get artificially with the 

 spectrum with which the sun presents us natu- 

 rally ; and the more we increase the tension — the 

 larger the coil, and the larger the jar we employ 

 up to a certain point — the more can we make our 

 terrestrial calcium vibrate in harmony, so to 

 speak, with the calcium which exists in the at- 

 mosphere of the sun. 



This gives us some very precious knowledge. 

 We know that to get things into harmony, as I 

 said before, we must employ a large induction- 

 coil ; and we know, again, that if we do employ 

 a large induction-coil, all the beautiful flutings in 

 the carbon-spectrum disappear utterly ; that kind 

 of carbon is no longer present in the reaction ; 

 instead of them we have a kind of carbon which 

 is only competent to give us bright lines ; and 

 we know that those bright lines do not exist 

 reversed in the spectrum of the sun, though the 

 channeled spaces do. Hence we assume the 

 carbon to lie in a cooler and therefore higher 

 region. 



In what I have written up to this point (and 

 I have just touched slightly on the physical side 

 of the work, because I believe that in the future 

 it will be most rich in teachings of the kind I 

 have indicated), I have dealt solely with the 

 Fraunhofcr — that is, the dark — lines in the solar 

 spectrum. Now, it is knowledge ten years old, 

 that if we observe the solar spectrum with that 

 considerable dispersion which is now impera- 

 tive if we are to do much good with it, there are 



bright lines in the ordinary solar spectrum side 

 by side with the dark ones. 



In a paper communicated to the Royal Socie- 

 ty, in 1868, attention was drawn to these bright 

 regions in the ordinary spectrum, and the posi- 

 tion of certain bright lines was stated. These 

 bright lines have since been rediscovered both by 

 Hennessy and Cornu. I will here call especial 

 attention to one line, because the requisite amount 

 of dispersion is now so generally available that 

 any one, whenever the sun shines, may turn to b f 

 the triple dark line in the green part of the spec- 

 trum, and see that bright line for himself. It 

 will be found just as much outside the fourth line 

 of b as the third is on the other side of it. This 

 bright line, lying in the most visible part of the 

 spectrum, is exactly similar to many others, some 

 of them in the yellow and some of them in the 

 red. A careful list of these lines was made by 

 myself some years ago, and I am sorry to say 

 the list was unfortunately left in a metropolitan 

 railway-carriage by one of my assistants ; at all 

 events, enough was said in this and other coun- 

 tries about these bright lines in the years 1869 

 and 1870 to have given rise, at all events, to the 

 hope that any one interested in solar physics 

 would be perfectly familiar with them. Among 

 other matters which called attention to the fact 

 of their existence was a correspondence which 

 took place in the Comptes-Rendus of the Academy 

 of Sciences in Paris, between Father Secchi and 

 another observer in connection with solar spots. 



I have insisted upon the fact that a large dis- 

 persion is requisite to see these bright lines, be- 

 cause with a small dispersion bright regions of 

 the sun are very apparent. These, however, are 

 due to the absence of fine lines ; and, indeed, if 

 one observes the solar spectrum with considera- 

 ble dispersion through a cloud which prevents the 

 fine lines being seen, then there is a very consid- 

 erable diminution in the intensity of some parts 

 of the spectrum, and a considerable relative in- 

 crease hi others, where these very fine lines are 

 present and absent respectively. When, how* 

 ever, considerable dispersion is employed, and 

 photography is brought into play, if precautions 

 be taken to give sufficient exposure, these bright 

 regions, as opposed to the bright lines, entirely 

 disappear. 



During the course of last year Dr. Draper, of 

 New York, published the first results of a re- 

 search which he has undertaken, going over very 

 much the same ground with regard to the metal- 

 loids as had been gone over in this country with 

 regard to the metals. Dr. Draper, who has long 



