Resonance Spectra of Sodium Vapour. 591 
The series are not as pronounced in the case of the zinc 
excitation as in many of the other cases. Apparently the 
series which I have marked " a " and which is excited by the 
longest of the three zinc lines, appears to have its upper 
members excited by one of the other zinc lines, and not by 
the line which starts the series. In other words, the line 
4883*6 seems to be the last line of the "a" series excited by 
Zn 4810" 7, but we find a number of lines in the spectrum 
excited by the total radiation of the zinc lamp, which form a 
continuation of this series. Neither of the other zinc lines, 
however, falls at points determined by extending the series 
down to their region. We must remember, however, that 
when we stimulate the vapour in the blue region, the series 
which starts at the point of stimulation gradually dies out 
as we go up the spectrum, and other series make their 
appearance, a circumstance which explains the apparent 
difficulty just mentioned. There seems to be some evidence 
that as we ascend the spectrum the spacing between the lines 
of a series begins to grow less again, that is the spacing- 
has a maximum value somewhere in the middle of the spec- 
trum. Compare the series " a " and " d " for example. Too 
much importance must not be attached to these tables of 
differences, however, as the lines which belong together have 
been determined only from the older spectrograms, which 
are very unsatisfactory, and by spacing off the new spectrum 
chart with a pair of compasses. The resonance spectrum has 
now been photographed with the 12- foot concave grating, 
and as soon as the new zinc lamps arrive the spectra excited 
by the isolated zinc lines will be recorded with the same 
instrument. 
Lithium Excitation. 
The light of the lithium arc stimulates a most remarkable 
group of resonance spectra. It is the only means that I have 
yet found of exciting definite series of lines in the red region 
of the fluorescence spectrum. There are four exciting lines 
in all, but they are so widely separated that the resonance 
spectra excited by them only overlap to a very slight degree. 
A photograph taken on a panchromatic plate of the complete 
spectrum excited by the lithium arc is reproduced on 
PI. XVIII. fig. 1. The points of excitation are indicated by 
arrows. 
The blue line (X = 4603) excites a resonance spectrum 
consisting of a nearly equidistant group of lines in the violet, 
and another group in the yellow-green. These two groups 
are indicated by the brackets " A" 
The blue-green line (\ = 4972) excites a remarkable series 
