TO SPECTROSCOPIC MEASUREMENTS. 15 



the experimental visibility ctu've of the red Hue near 6439, coiTected for 

 personal equation, together with the simple exponential curve 



The remarkably close agreement leaves no doubt that the distribution of 

 hght in the source follows very nearly the exponential law giving the curve 

 in Fia-. lOrt, in which the " half -width " of the source is 0.0065. 



The results of a single set of observations on the gi-een hne at 5086 is 

 given in Fig. 11&, Plate III., the approximate agreement between the full 

 line and the dotted curve, which con-esponds to the equation 



V=2~^'''"'' ms 0.2 115, 

 showing that the source is a close double, the intensity of whose compo- 

 nents is in the ratio 5:1, and whose distance apart is 0.022 ; the "half -width" 

 of each component being 0.0018. 



The cmwe for the blue radiation at 1800 is given in Fig. 12&, Plate III., 

 and shows that the results may be approximately represented by 



^=2"^'"*' cos 0.1/32, 

 which corresponds to the distribution of intensity given in Fig. 12o. 



Thallium. The metal is not suflaeiently volatile at the temperatures 

 attainable, but the chloride answers admirably, giving a briUiant green 

 light, the visibihty curve varying but little with temperatm-e. This cm"ve 

 is given in Fig. 13&, Plate III., together with the dotted curve representing 

 the equation 



V=^€OS 0.2 160 a/ 4 Fi'' + V,'+4: V, V, cos 2 xX/25.3, 



in which Fi = 2 '' 



and y, = 2-^"--. 



This is the visibihty ciu've due to a double source, each of whose com- 

 ponents is a close double, as shown in Fig. 13a. 



Mercury. Mercury in a vacuum-tube gives two yeUow lines 5790 and 

 5770, a very bi-illiant green line at 5461, and a violet hne at 4358. 



The yellow hues are not very bright, and are so close together that it is 

 somewhat difacult with the dispersion employed to prevent the hght fi-om 



