296 Mr. A. A. Michelson on the Application of 



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 sub- 

 stance might of itself affect 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 hydrogen. The character of the visi- 

 bility-curve was not perceptibly altered. 



In the same series of experiments it was found that, pro- 

 vided 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 connexion it may be 

 mentioned that the character of the curve for the green mer- 

 cury-line was not essentially altered when, in place of metallic 

 mercury, the nitrate, iodide, or the chloride was substituted, 

 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 illus- 

 trated by the curves in figs. 21a and 21b, Plate VII. 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 dis- 

 tances apart, and their relative intensities 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 curves for sodium, thallium, and lithium in 

 the flame of a Bunsen-burner have been observed ; and the 

 results are given in fig. 22, Plate VII. The thallium- and 

 lithium-lines are clearly double ; the distance between the 

 components of the former agreeing very well with the results 

 obtained with the vacuum-tube. 



These substances were brought into the flame in the ordi- 

 nary 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 



