Interference Methods to Spectroscopic Measurements. 289 



From these data fig. 3 a, Plate V., was constructed, the 

 full curve showing the distribution of light in the source. 



Fig. 4b, Plate V., gives, in the full curve, the corrected 

 values of the visibility of the blue hydrogen-line, at the same 

 temperature and pressure as before. The dotted curve repre- 

 sents a double exponential, as before. The formula for this 

 curve is 



V=2- X2/242 cos -7/28, 



thus giving « = 0'08forthe distance between the components, 

 and S = 0'057 for the " half-width " of each. These values 

 give for the distribution of light in the blue hydrogen-line, the 

 full curve in fig. 4 a. 



Oxygen, 



Fig. 5, Plate V., represents the results obtained from 

 oxygen prepared by heating a tube containing mercuric oxide, 

 drying the gas by sulphuric acid, and exhausting and filling 

 repeatedly till the spectrum was nearly pure. The lines are 

 much less bright than those of hydrogen ; and in order to 

 obtain satisfactory results, the current had to be increased so 

 far that the tube was frequently broken. Notwithstanding 

 the somewhat uncertain character of the observations, it will 

 be seen from fig. 5a that the curve for the orange-red line cor- 

 responds very well with that given by the formula 



V=2 _X2/342 [-36 + -32 cos 2ttX/2-69 + -16 cos 2ttX/4-85 



+ -16C0S27TX/1-73] 1 - 



The agreement between the coefficient 2~ x ~ /34 and the general 

 curve drawn through the maxima is also shown in fig. 5b, 

 Plate V. • 



The interpretation of these results is that the orange-red 

 oxygen line is a triple, whose components have intensities in 

 the ratios 1:1: 1/2, and whose distances apart are 1*51 and 

 0-84 respectively, and whose " half-width " is 0'027. This is 

 shown in fig. 5c. 



Sodium. 



The results obtained from metallic sodium in the vacuum- 

 tube are so varied, the character of the lines being so consi- 

 derably altered by temperature and pressure, that a complete 

 study is at present impossible. This is especially true of the 

 yellow lines.; and the difficulty is considerably increased on 

 account of the insufficiency of the dispersion used, which does 

 not permit the separate examination of the lines. Some 

 reference to the changes mentioned will be given at the close 



