592 Prof. R. W. Wood on the 
of lines midway between the groups " A." This series is of 
especial interest, for, as will appear presently, the barium 
line 4934 takes hold of the same series, at the first line below 
the exciting line of the lithium series. The red and orange 
lines of lithium also excite two well-marked series in the red 
region of the spectrum, the individual lines of which appear 
to be very nearly equidistant. As yet I have not determined 
the wave-lengths of these lines, though they can be determined 
at any time from the plate which has an iron comparison 
spectrum running parallel to the resonance spectra. Taken 
as a whole, this plate is the most beautiful illustration of 
resonance spectra excited by monochromatic stimulations, 
that I have obtained up to the present time. The photograph 
was made with a small two-prism spectrograph, and has not 
been used for wave-length determinations owing to the small 
dispersion and the great width of the slit. Spectrograms 
best suited to the accurate measurement of the lines do not 
reproduce very well, consequently I have used only those 
taken with a wide slit for purposes of illustration, and the 
possible accuracy of the wave-length determinations must 
not be judged from the photographs which accompany 
this paper. 
AYe will now consider the series excited by the green-blue 
line 4972. The wave-lengths of the lines in this series are 
recorded in the following table. The wave-lengths I consider 
accurate to about three tenths of an Angstrom unit, as the 
plate was an especially good one, taken with a very fine slit. 
Differences. 
4862-0* oo. q 
4S96-2* ™* 
49340* ^.V 
4972-3 * 6yi 
?°°7'4 39 . 2 
r»009'b 
oOiri* oq.q 
5049-9* d °-' 
5085-9 39-4 
5089-1 * 
5117*8 oo.o 
51237 683 
5128-2* 
5152 9 o7.q 
5162-4 d ' S 
51672* 
5191-8 oi. 9 
51994 6iZ 
5207-1 * ? 
5229-6 * * o-.o 
52669** ?'* 
53043** ot "* 
