312 THE CHEMISTRY OF THE SUN. [CHAP. 



I may begin by stating the way in which the observations 

 have been made. The work, which has been in large part 

 done by my assistants and chiefly by Messrs. Lawrance and 

 Greening, simply consists of a survey of the two regions F I 

 and b D. 



The most widened line in each region not the widest line, but 

 the most widened, is first noted ; its wave-length being given in 

 the observation books from Angstrom's map. Next the lines 

 which most nearly approach the first one in widening are recorded, 

 and so on till the positions of six lines have been noted, the 

 wave-lengths being given for each region, from Angstrom's map. 



It is to be observed that these observations are made without 

 any reference whatever to the origin of the lines ; that is to say, 

 it is no part of the observer's work to see Avhether there are 

 metallic coincidences or not ; this point has only been inquired 

 into in the reductions. In this way perfect absence of all bias 

 is secured. It may further be remarked that the number of 

 lines widened at any time during a sun-spot period is about the 

 same, so that the conditions of observation vary very little from 

 month to month, or from year to year. 



The absolute uniformity of the results obtained in the case of 

 the chemical elements investigated indicates, I think, that the 

 observations have been thoroughly well made ; and, as a matter 

 of fact, they are not difficult. 



It was not only important to record these observations for 

 their own sake, but also for the comparison they allowed us to 

 make with other observations of different phenomena, especially 

 of those presented by the prominences. Side by side with the 

 sun-spot work, therefore, went on an elaborate mapping of every 

 prominence line observed by Tacchini since 1870. The reason 

 of this is obvious, but will bear repetition here. The spots, as 

 everybody agrees, are caused by down currents when a disturbance 

 in the solar atmosphere brings vapour down from the cooler ex- 

 terior regions. The prominences, on the other hand, are either 



