Resonance Spectra of Sodium Vapour. 589 
Zinc Excitation. 
The wave-lengths in the resonance spectrum excited by 
the three blue zinc lines have been determined to within less 
than 0*2 A.E. Two negatives were made with the large 
grating and Cooke long-focus lens, one with an exceedingly 
narrow slit, which was used for the measurements, and one 
with a wider slit, which is reproduced on PL XVIII. fig. 2 : 
the white-light fluorescence above, iron comparison between. 
On this Plate the lower end of the spectrum did not appear 
owing to the fact that a corner of a black cloth inside the 
spectrograph slipped down in front of the end of the plate. 
The reproduction gives a good idea of the sharpness of the 
lines in the resonance spectrum, although the exciting lines 
do not appear on the plate. It must be remembered too 
that this plate was made with what I have termed a wide 
slit. 
The resonance spectrum contains a large number of series 
of equidistant lines, though the series are not as extended as 
in the case of the cadmium excitation and many others 
which will be considered presently. 
I have not, at the present time, photographed the spectra 
excited by the three lines separately, but by comparing the 
plate made with the grating with the one made in the earlier 
work, I have made a provisional assignment of the lines 
which I believe belong to the spectrum excited by the longest 
of the three zinc lines. These lines I have marked with a 
star in the Table of wave-lengths, and have designated with 
letters a, b, c, &c, the lines which appear to belong to the 
same series. 
As many lines appear double in the present photographs 
which appeared as single lines in the earlier spectrograms, 
it will be understood that too much importance must not be 
attached to the present assignment of the lines. Many of 
the series appear to have missing lines, and an attempt is 
now being made to find them with longer exposures. 
Exposures of 8 or 10 hours were necessary with the quartz 
zinc lamp fed by a current of 5 amperes, and the lamps are 
of very little use after a run of 25 or 30 hours, owing to a 
black deposit (reduced silica probably) which forms on the 
inner surface of the tube. 
The wave-lengths follow, together with the differences of 
wave-length of the lines which appear to belong to the same 
series. The exciting lines are underscored, and the relative 
intensities are roughly indicated by numerals, 10 meaning 
very strong, and 1 very faint : — 
