BAND OF AMMONIA, AND ITS OCCURRENCE IN THE SOLAR SPECTRUM. 355 
In the arc, the central and secondary maxima are more fully developed than in the 
flame, in the sense that the band lines extend to greater distances from the two 
heads. That the majority of the additional lines are really due to ammonia, and 
have not been introduced from other possible sources in the arc, is sufficiently proved 
by the series investigation, which shows that many of them are associated with the 
lines which appear in the flame. This greater development of the central parts of 
the band in the arc is accompanied by a weakening of the groups of three in the 
vicinity of the maxima, and a relative intensification of those further away. 
Similar modifications have also been noted in the case of vacuum tube spectra 
when discharges of different intensity have been employed. No special experiments 
have been made in this connection, but in some photographs of the spectrum of 
nitrogen where ammonia appears as an impurity, it has been observed that the groups 
of three near the central maxima are developed relatively strongly by feeble 
discharges, while strong discharges enhance the groups away from the maxima. 
Increased intensity of discharge thus appears to produce the same change of 
spectrum as the increase of temperature in passing from the flame to the arc. 
Estimates of the intensities of the lines in both flame and arc have accordingly 
been included in the general list (Table V.), but the comparison of the two sources 
in the region of the central maximum is incomplete on account of the smaller 
resolution in the photographs of the flame spectrum. 
The wave-lengths of the lines were determined in the usual manner by inter¬ 
polation with respect to lines of iron, as given by Burns. Lines of nickel originating 
in the poles employed for the arc spectrum, for which wave-lengths are also given by 
Burns, served for the detection of small displacements of the reference spectra, and 
to indicate the corrections to be applied. Most of the lines could be measured on the 
grating plates of the arc spectrum, but some were obscured by lines due to the poles, 
and their positions were necessarily determined from the quartz Littrow photographs 
of the flame spectrum. It is hoped that in most cases the wave-lengths are accurate 
to within O'OlA. 
Details of the wave-lengths and intensities are included in Table V. 
Structure of the Band. 
The Groups of Three .—A considerable amount of regularity in the structure of 
the ammonia band is obvious by mere inspection of the photographs. This is 
especially the case with regard to the groups of three, which extend for more than 
70A on each side of the central maximum. There are three series on the less 
refrangible side which coalesce towards the red, and three on the more refrangible 
side which coalesce towards the violet. To facilitate discussion, those on the less 
refrangible side have been designated a, /3, y, and those on the more refrangible side 
3, e, L in order of increasing refrangibility in the groups of three in each case. 
