ON SPECTROSCOPIC MEASUEEMENTS. 179 



V=2~'^°'^^^' COS -2/115) showing that the source is a close double, the 

 intensity of whose components is in the ratio 5:1, and whose distance 

 apart is' '022, the 'half-width ' of each component being 0-0048. 



The curve for the blue radiation at 4800 is given in fig. 126, Plate 

 [I., and shows that the results may be approximately represented by 

 V=2~^°'^* cos -1/32, which corresponds to the distribution of intensity 

 given in fig. 12a. 



Thallium. 



The metal is not sufficiently volatile at the temperatures attainable, 

 but the chloride answers admirably, giving a brilliant green light, the 

 visibility cui-ve varying but little with temperature. This curve is 

 given in fig. 136, Plate II., together with the dotted curve representing 

 the equation 



V=i cos -2/160 n/4V,2 + V22+4ViV2 cos 27rX/25-3, 

 in which 



and 



This is the visibility curve due to a double source, each of \\ hose 

 components is a close double, as shown in fig. 13a. 



Mercury. 



Mercury in a vacuum tube gives two yellow lines, 5790 and 5770, a 

 very brilliant green line at 5461, and a violet line at 4358. 



The yellow lines are not very bright, and are., so close together that it 

 is somewhat difficult with the dispersion employed to prevent the light 

 from overlapping. Notwithstanding these difficulties, the close agree- 

 ment of a number of observations shows that the curve for the lower line, 

 given in fig. 146, Plate II., is a close approximation to the truth. 

 Neglecting the effect of a line of feeble intensity at a distance of about 

 ■24 from the principal line, the distribution of light in the source is 

 represented in fig. 14a, which gives for the visibility curve 



in which 

 and 



V=i>/3V,2 + V22 + 6YjY^ cos 27rX/28, 



^_2-X7200»^ 



¥2=2"^°'''°' COS -5/280. 



Fig. 156, Plate II., represents the results of observations on the 

 upper yellow line, omitting some peculiarities due to the presence of one 

 or naore lines of feeble intensity. The curve agrees closely with the 

 formula 



in which 

 and 



V=Jn/3V,2+\V + 6ViV2 cos 27rX/70, 



V,=2-^"'^^^ 



N 2 



