180 EEPOKT— 1892. 



which, represents the visibility curve produced by two lines of intensities, 

 1 : 3 and separated by 0-019 division as shown in fig. 15a. 



The green mercury line is one of the most complex yet examined. 

 The constituent lines are, nevertheless, so fine that the interference bands 

 are frequently visible when the difference of path is over four-tenths of 

 a metre. The full curve in fig. 1€6, Plate II., gives the results of 

 observations corrected for personal equation, while the dotted curve 

 represents the equation 



V=2~''^^''^^V-69Vi2 + -03V2^+^8V,V2 cos 27rX/31-4, 

 in which 



V,=-62 + -38 cos 27rX/360, 

 and 



V2=77 + -23 cos 27rX/110. 



This is the visibility curve corresponding to the distribution repre- 

 sented in fig. 16a. The components of the line, for simplicity, have been 

 assumed to be symmetrical, as figured ; but the observations are not 

 sufficiently accurate to determine whether, for instance, each component 

 is a double or a triple line. In this case, also, as in the preceding ones, 

 it is impossible, from the data given, to determine whether the smaller 

 component is to the right or left of the principal line. . A direct obser- 

 vation with the grating showed, however, that the smaller component is 

 towards 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 

 line, and whose distance from it is about three times that of the chief 

 components. 



The violet mercury line is much more difficult to observe than the 

 others. The results obtained by observation, corrected for personal 

 equations, are given by the full curve fig. 176, Plate II. The formula 

 for the dotted curve is 



in which 

 and 



V=n/ •88V,2-i--12ViV2 cos 27rX/23, 

 V,=2"^''^*' [-62-1- -38 cos 27rX/200], 



* 2 — - ) 



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



The results of the preceding work are collected for comparison in 

 fig. 18, Plate III., together with the D group in the solar spectrum. 

 From these, as well as from the curves, it will be seen that it is easy by 

 this method to separate lines whose distance apart is only a thousandth 

 of that between D, and Dj, and even to determine the distribution of 

 light in the separate components. The conditions most favourable 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 slight variations of the conditions 

 shows that the behaviour of each substance must be carefully studied 

 tinder all possible circumstances of temperature, pressure, strength of 



