ON SPECTROSCOPIC MEASUREMENTS. 177 



If A be the value of X for V=^, then 8= — -, or, with sufficient ac- 



. -22 

 curacy, o=— , 



Substituting the value of 8 in the equation for V, we have V=2 ^• 

 The value of A in the hydrogen carve is 19. Accordingly, after reducing 

 to the same units as above, we have 8=0"049. 



From these data fig. 3a was constructed, the full curve showing the 

 distribution of light in the source. 



Fig. 4&, Plate I., gives, in the fall curve, the corrected values of 

 the visibility of the blue hydrogen line, at the same temperature and 

 pressure as before. The dotted curve represents a double exponential, as 

 before. The formula for this curve is 



V=2~^°'^*' cos -7/28, 



thus giving a=0'08 for the distance between the components, and 

 8=0'057 for the ' half-width ' of each. These values give for the distribu- 

 tion of light in the blue hydrogen line the fall curve in fig. 4a. 



Oxygen. 



Fig. 5, Plate I., 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 hydi'ogen, and in 

 order to obtain satisfactory results,\the current had to be increased so 

 far that the tube was frequently broken. Notwitbetanding the somewhat 

 uncertain character of the observations, it wlir be seen from fig. 5a that 

 the curve for the orange-red line corresponds very well with that given 

 by the formula 



V=2~^''^*"[-36 + -32 cos 27rX/2-69 + -16 cos 27rX/4-85 



+ -16cos27rX/l-73]*. 



The agreement between the coefficient 2 and the general curve 



drawn through the maxima is also shown in fig. 6&. 



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. 



Sodu 



lum. 



The results obtained from metallic sodium in the vacuum tube are so 

 varied, the character of the lines being so considerably altered by tem- 

 perature and pressure, that a complete study is at present impossible. 



This is especially true of the yellow lines, and the difficulty is con- 

 siderably 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 of this 



T?92. N 



