Interference Methods to Spectroscopic Measurements. 297 



at 1500° C. and that in the vacuum-tubes at 350° C, the lines 

 should be onty twice as broad in the former case as in the 

 latter. It appears, then, that notwithstanding the small 

 quantity of substance present (barely enough to colour the 

 flame), the real density must be comparable to that of the 

 vapour of the substance boiling under atmospheric pressure. 



The principal object of the foregoing work is to illustrate 

 the advantages which may be expected from a study of the 

 variations of clearness of interference-fringes with increase in 

 difference of path. The fundamental principle by which the 

 " structure " of a line or group of lines is determined by this 

 method is not essentially different from that of spectrum- 

 analysis by the grating, both depending, in fact, on interference 

 phenomena ; but in consequence of the almost complete 

 freedom from errors arising from defects in optical or mecha- 

 nical parts, the method has extraordinary advantages for this 

 special work. A glance at fig. 18, Plate VIII., will give a 

 fair idea of the (t resolving-power " of the method as compared 

 with that of the grating. In order that the comparison be 

 quite fair, however, it would be necessary to take for a com- 

 parison-spectrum that of the substances here used, and under 

 the same conditions. With the best instrumental appliances 

 now in use, it is difficult to " resolve " lines as close together 

 as the components of either of the yellow sodium-lines. It is 

 evident, however, that by Light-ivave Analysis (if I may 

 venture so to call the foregoing method) a tenth of this dis- 

 tance is obviously within the limit ; indeed, if the width of the 

 lines themselves be less than their distance apart, there can 

 be no limit. 



Supplement. 



I. It has already been pointed out that in many cases it is 

 difficult or impossible to decide between two or more distri- 

 butions of lines which give very nearly the same visibility- 

 curve ; and when there are many lines in the source, the 

 combinations of intensities and arrangement of these from 

 which a type may be selected is enormously great. Indeed, 

 even when the number of lines is greater than three, except- 

 ing perhaps the cases where the lines may be in pairs (as in 

 the case of yellow sodium-light), the resulting visibility-curve 

 becomes so complex that it is very difficult to analyse. Doubt- 

 less in many cases where the components are not too close, the 

 grating will give the information necessary for the investi- 

 gator to select the proper combination. 



It may readily be shown that the formula 



y — p2 ' 



