TO SPECTROSCOPIC MEASUREMENTS. 21 



mercmy, the nitrate, iodide, or the chloride was substituted, the only impor- 

 tant effect being a diminution in the visibility in the order named. 



In the case of yellow sothuni light, it has already been mentioned that 

 the character of the curve is more variable than that of any other Hne thus 

 far examined. This is illustrated by the curves in Fig. 21a, and Fig. 216, 

 Plate V. It has not been possible thus far to devote the attention which 

 a systematic investigation demands. These changes are very puzzling to 

 trace, but undoubtedly much of the cUfiQculty is due to the fact that the dis- 

 persion employed was not sufficient to permit the separate examination of 

 the components. Still there can be no doubt that the width of the lines, 

 theu' distances apart, and their relative intensities vary rapidly with changes 

 in temperature and pressure. 



In addition to the preceding investigations of visibihty curves for light 

 emanating from a rare gas or vapor in a vacuum-tube, the curves for sodium, 

 thallimn, and htliium in the flame of a Bunsen l)iu-ner have been obsei-ved, 

 and the results are given in Fig. 22, Plate V. The thallium and lithium hnes 

 are clearly double, the distance between the components of the former agi'ee- 

 ing very well with the results obtained with the vacuum-tube. 



These substances were brought into the flame in the ordinary way, and 

 the results obtained were at least as good as when a finely divided solution 

 was used according to the method of Grouy. It appears from these curves 

 that the width of the line is about ten times as great as when the vacuum- 

 tube is iised. But if the temperatm-e of the flame be taken at 1500° C, 

 and that in the vacuum-tubes at 350° C, the lines should be only twice as 

 broad in the former case as in the latter. It appears then that notwith- 

 standing the small quantity of substance present (barely enough to color 

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

 substance boiling under atmospheric pressure. 



The principal object of the foregoing work is to illustrate the advan- 

 tages which may be expected fi'om a study of the variations of clearness of 

 interference fringes with increase in difference of path. The fimdamental 

 principle by which the " structure" of a line or group of hnes is determhied by 

 this method is not essentially different from that of spectinun analysis l)y 

 the grating, both depending in fact on interference phenomena ; but in con- 

 sequence of the almost complete freedom fi'om eiTors arising fi'om defects 

 in optical or mechanical parts, the method has extraordinary advantages for 

 this special work. A glance at Fig. 18, Plate IV., will give a fair idea of the 



