178 REPORT — 1892. 



paper. At present it will suffice to take a particular case, the pressure 

 being very low and the temperature about 250°.' 



The full curve in fig. 6b, Plate I., gives the experimental result for 

 the visibility at the maxima for yellow sodium light, corrected for the 

 personal equation. The dotted curve corresponds to the formula 



V=2~^'^^'' cos -7/50 cos -1/140. 



The complete equation, assuming that the two lines are alike, is 



Y=2~^''^^^' cos •8/Q-58 cos -7/50 cos •1/140. 



The interpretation of these results is that each of the sodium lines is 

 a close double, as shown in fig. 6a. 



The yellow-green sodium-line at A.= 5687 is a double whose com- 

 ponents are about the same distance apait as the yellow pair. It was 

 found to be far less variable than the yellow ; and the full visibility 

 curve, neglecting slight irregularities, gives the experimental results 

 corrected for personal equation. Fig. 7b, Plate I., shows that its com- 

 ponents are single, and correspond in distribution of light fairly well 

 with the exponential curve, fig. 7a. 



The same may be said of the orange-red double at 6156 also, except 

 that this seems to have a companion of feeble intensity. 



The doubles at 5150 and at 4982 were also examined, the curves 

 showing nearly the same results as the red. 



Zinc. 



The temperature at which the radiations from metallic ziuc could be 

 conveniently observed was in the neighbourhood of the melting-point of 

 the glass of which the vacuum tubes were made. But few observations 

 were recorded, though these were quite consistent. The results of the 

 observations, corrected for personal equation, are given in figs. 8 and 9, 

 Plate I. The former is the record obtained from the red line near 

 6360, and shows that this line is single, the distribution of light agreeing 

 very well with a simple exponential curve, the ' half-width ', being 0013. 

 The latter shows the results of observation on the blue line near 4811. 

 The dotted curve is the visibility curve due to a distribution represented 

 in fig. 9a. 



Cadmium. 



Metallic cadmium in the vacuum tube at a temperature of about 280° 

 gives a number of very bright lines, widely separated, and varying very 

 slightly with temperature or pressure. Fig. 106, Plate II., shows the 

 experimental visibility curve of the red line near 6439, corrected for the 



personal equation, together with the simple exponential curve V=2 

 The remarkably close agreeruent leaves no doubt that the distribution of 

 light in the source follows very nearly the exponential law, giving the 

 curve in fig. 10a, 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. lib, Plate II., the approximate agreement between the 

 full line and the dotted curve (which corresponds to the equation 



' The curve given above was obtained a year ago ; and since then it has been 

 impossible to reproduce it exactly. 



