xvi.] GENERAL STATEMENT. 225 



which gives us A compared with B we have so many lines of the 

 two substances. Now we say if B exists in A as an impurity, 

 the longest line of B will be there. We look for the longest 

 line of B, and we find it, and we put a minus sign over that line 

 in A to show it is most probably due to an impurity of B. We 

 then ask if there are any more lines of B in A, and we naturally 

 look for the next longest line of B ; we find that, and we put a 

 minus sign over that ; and then we look for the next longest line 

 and mark that ; then we look for the next one ; it is not there 

 then there are no more lines of B in A. In that way, if we 

 knew everything, we should years ago have been able to map a 

 spectrum of a substance A, from which all traces of the spectro- 

 scopic effects due to the presence of a substance B, had been 

 eliminated, and we might go on with substance c, and so 

 on ; and in that way eliminate the effects produced by c as well 

 as by B from the spectrum of the substance A. 



Lines due to impurities, thus traced, were marked for omission 

 from the map and their true sources recorded, while any line 

 that was observed to vary in length and thickness in the various 

 photographs was at once suspected to be an impurity line, and 

 if traced to such was likewise marked for omission. 



For instance, the longest line to the left in c appears as a short 

 line in A.. It is therefore omitted from A. The three longest 

 lines in B appear in A. They are therefore omitted from A. 

 While the shortest line in B, which also appears in A, is marked 

 as a common line, because the lines of intermediate length are 

 not represented in A. 



The retention or rejection of lines coincident in two or more 

 spectra is determined, then, by observing in which spectrum the 

 line is longest and thickest. Where several elements were mapped 

 at once, all their spectra were confronted on the same plate, as 

 this means the presence of one of the substances as an impurity 

 in the others could be at once detected. 



Q 



