Interference Methods to Spectroscopic Measurements. 291 



Cadmium. 



Metallic cadmium in the vacuum-tube at a temperature of 

 about 280° gives a number of very bright lines, widely sepa- 

 rated, and varying very slightly with temperature or pressure. 

 Fig. 10 b, Plate VI., shows the experimental visibility-curve 

 of the red line near 6439, corrected for the personal equation, 

 together with the simple exponential curve V= 2 -x /138 . The 

 remarkably close agreement leaves no doubt that the distri- 

 bution of light in the source follows very nearly the expo- 

 nential law giving the curve in fig. 10 a, in which the " half- 

 width " of the source is 0*0065. 



The result of a single set of observations on the green line 

 at 5086 is given in fig. 11 b, Plate VI., the approximate 

 agreement between the full line and the dotted curve (which 

 corresponds to the equation V = 2~ X2/12 ° 2 cos -2/115) showing 

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

 ponents 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. 12 5, Plate VI., and shows that the results may be 

 approximately represented by V = 2 -X2/642 cos -1/32, which 

 corresponds to the distribution of intensity given in fig. 12 a. 



Thallium. 



The metal is not sufficiently volatile at the temperatures 

 attainable, but the chloride answers admirably, giving a 

 brilliant green light, the visibility-curve varying but little 

 with temperature. This curve is given in fig. 13 b, Plate VI., 

 together with the dotted curve representing the equation 



Y = ± cos -2/160 v / 4V 1 2 + V 2 2 + 4V 1 V 2 cos 2ttX/25*3, 



in which V 1 = 2" x ^ 462 and V 2 = 2" X2/1882 . 



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

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



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. Notwith- 

 standing these difficulties, the close agreement of a number 

 of observations shows that the curve for the lower line, given 



