TO SPECTROSCOPIC MEASUREMENTS. IT 



the data given to determine wliether tlie smaller component is to tlie right 

 or left of the principal line. A direct observation with the grating showed, 

 however, that the smaller component is toward the red end of the spectrum. 



The full curve shows that there is at least one other line — probably 

 more than one — whose intensity is roughly one twentieth of the principal 

 hne, and whose distance fi'om it is al)out three times that of the chief 

 components. 



The violet merciuy line is much more tUfficult to obsein^e than the 

 others. The results obtained by observation, corrected for personal equa- 

 tion, are given by the ftdl curve Fig. lib, Plate III. The formula for the 

 dotted curve is 



V=a/ 0.88 Fi^+0.12 V, V, co^2r.X/23, 



in which Fi = 2 ^''' 



74 2 



0.62+0.38 cos 2 T.X2Q0 



and V, = 2 



-X'i, 



1202 



5 



the resulting distribution of light shown in Fig. 17a. 



The results of the preceding work are collected for comparison in Fig. 18, 

 Plate IV., together with the D group in the solar spectrum. From these, as 

 weU as from the curves, it wiU be seen that it is easy by this method to 

 separate lines whose distance aparLis only a thousandth of that between 

 Di and D^, and even to detennin^ne distribution of light in the separate 

 components. The conditions n|pst favorable to high values of the visibility 

 are low density and low temperature, and these conditions were complied 

 with as far as possible. Still, in many cases, the range of visibility due to 

 sUght variations of the conditions shows that the behavior of each substance 

 must be carefully studied under aU possible cu-cumstances of temperature, 

 pressure, strength of current, size and shape of the electrodes, diameter of 

 the vacuum-tube, etc. 



The effect of temperatiu-e and of pressure on the visibiHty may be readily 

 accounted for on the kinetic theory. In fact, there is l)ut little doubt that 

 these are the chief if not the sole causes of the broadening of the spectral 

 lines and the consequent diminution of visibihty ; the latter cause acting by 

 altering the period of the source by frequent colhsions, and the former by 

 the change in the wave-length of the hght due to the motion of the source 

 in the hne of sight. 



