ON SPECTKOSCOPIC MEASUREMENTS. 183 



same ; but it may be important to note that there are a number of excep- 

 tions to this rule, among which the green mercury line and the yellow 

 sodium line may be especially mentioned. 



Thus, fig. 20a, Plate IV., represents the visibility curve usually 

 observed for the green mercury line, and fig. 20c represents that obtained 

 when the vacuum is so high that the discharge passes with difficulty, 

 while fig. 206 represents the intermediate stage. This last observation 

 was obtained by placing the mercury in an atmosphere of hydrogen whose 

 pressure could be measured by a McLeod gauge. 



It might be objected that the presence of a foreign substance might 

 of itself aifect the distribution of light in the source, and therefore the 

 form of the curve. In order to test this point, a series of observations of 

 the red hydrogen line was taken, while the tube contained liquid mercury, 

 which was heated until the mercury spectrum was at least ten times as 

 bright as that of the hydi'ogen. The character of the visibility curve was 

 not perceptibly altered. 



In the same series of experiments it was found that, provided the 

 pressure of the hydrogen remained constant, the effect of a change in 

 temperature from 75° to 140° had no appreciable effect on the result. In 

 this connection it may be mentioned that the character of the curve for 

 the green mercury line was not essentially altered when, in place of 

 metallic mercury, the nitrate, iodide, or the chloride was suljstituted, the 

 only important effect being a diminution in the visibility in the order 

 named. 



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

 the character of the curve is more variable than that of any other line 

 thus far examined. This is illustrated by the curves in fig. 21a and 

 fig. 21b, Plate IV. 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 difficulty is due to 

 the fact that the dispersion employed was not sufficient to permit the 

 separate examination of the components. Still, there can be no doubt 

 that the width of the lines, their distances apart, and their relative inten- 

 sities vary rapidly with changes in temperature and pressure. 



In addition to the preceding investigations of visibility curves for 

 light emanating from a rare gas or vapour in a vacuum tube the cui-ves 

 for sodium, thallium, and lithium, in the flame of a Bunsen burner, have 

 been observed, and the results are given in fig. 22, Plate IV. The thal- 

 lium and lithium lines are clearly double, the distance between the com- 

 ponents of the former agreeing 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 Gouy. It appears from 

 these curves that the width of the line is about ten times as great as 

 when the vacuum tube is used. But if the temperature 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 notwithstanding the small quantity of substance 

 present (barely enough to colour the flame) the real density must be com- 

 parable to that of the vapour of the substance boiling under atmospheric 

 pressure. 



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



