xii.] A PRELIMINARY SEARCH. 157 



would have been of course necessary, but it was not absolutely 

 essential for the purposes of a preliminary inquiry to wait for 

 such a complete set of maps, since we had already learned that 

 the lists of lines given by the various observers may be made to 

 serve as a means of differentiating between the longest and 

 shortest lines, because, as we have seen, the lines given at a 

 low temperature, by a feeble percentage composition, or by a 

 chemical combination of the vapour to be observed, are pre- 

 cisely those lines which appear longest when the complete 

 spectrum of the pure dense vapour is studied. 



Now with regard to the various lists and maps published by 

 various observers, it was known (1) that very different tem- 

 peratures were employed to produce the spectra, some investi- 

 gators using the electric arc with great battery-power, others 

 the induction-spark with and without the jar ; (2) that some 

 observers employed in certain cases the chlorides of the metals 

 the spectra of which they were investigating, while others 

 used the metals themselves. 



It is obvious, then, that these differences of method could not 

 fail to produce differences of result ; and accordingly, in refer- 

 ring to various maps and tables of spectra, we find that 

 some include large numbers of lines omitted by others. A 

 reference to these tables, in connection with the methods em- 

 ployed, shows at once that the large lists are those of observers 

 using great battery-power or metallic electrodes, the small ones 

 those of observers using small battery-power or the chlorides. 

 If the lists of the latter class of observers be taken, we shall 

 have only the longest lines, while those omitted by them and 

 given by the former class will be the shortest ones. 



In cases, therefore, in which I had not then already (1873) 

 mapped the spectrum in the new way, I in the first instance 

 took the longest lines as thus approximately determined. 



A preliminary search having been determined on, I endea- 

 voured to get some guidance by seeing if there was any quality 



