TO SPECTROSCOPIC MEASUREMENTS. 13 



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

 formula for this curve is 



F=2"^'-*' cos 0.7/28, 



thus giving a = 0.08 for the distance between the components, and 5 = 0.057 

 for the " half -width " of each. These values give for the distribution of light 

 in the blue hydi'ogen hne the full cm-ve in Fig. 4a. 



Oxygen. Fig. 5, Plate II., represents the results obtained from oxygen 

 prepared by heatmg a tube containing mercm*ic oxide, diying the gas by 

 sulphuric acid, and exhausting and filling repeatedly, till the specti-um was 

 nearly pure. The hues 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 corresponds very well with that given by 

 the formula 



F=2~'^'''''' rO.36 + 0.32 cos 2 7rX/2.69 + 0.16 cos 2 irX/4.85+ 0.16 cos2 xX/1.73 ^ 



The agreement between the coefficient 2~^'"', and the general curve drawn 

 thi-ough the maxima, is also shown in Fig. 5&, Plate II. 



The interpretation of these results is that the orange-red oxygen hne is a 

 triple, whose components have intensities in the ratios 1:1: V2, and whose 

 distances apart are 1.51 and 0.84 respectively, and whose "half -width" is 

 0.027. This is shown in Fig. 5c. 



Sodmm. The results obtained from metaUic sodium in the vacuum-tube 

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

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

 is especially true of the yellow hues, and the difficulty is considerably in- 

 creased 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 ^dll be given at the close of this paper. At present it 

 win suffice to take a particular case,— the pressm-e being very low, and the 

 temperature about 250°.* 



* The curve given above was obtained a year ago, and since then it has been impossible to reproduce 

 it exactly. 



