582 Prof. ft. W. Wood on the 
still more accurate study of it advisable, and this work has 
been carried on by Mr. Clinkscales, one of my students, 
during the past year. Photographs taken with the 21-foot 
grating in the second-order spectrum, show that it is even 
more complicated than was originally believed. We find, 
on the average, from 60 to 70 absorption-lines within a space 
only 12 Ang. units in width ; in other words, as many as 
30 lines in a region no wider than the distance between the 
D lines. This means that in the blue-green channelled 
absorption spectrum, which is about 1200 A.E. in width, 
there are roughly speaking about 6000 absorption - lines. 
Taken collectively these lines form themselves into a number 
of groups, which resemble the groups seen in the absorption 
spectra of iodine and bromine, and in certain banded emission 
spectra. 
A small portion of the absorption spectrum of sodium 
vapour in vacuo, taken with the 21-foot grating in the 
second-order spectrum by Mr. Clinkscales, is reproduced on 
PL XIX. figs. 4 & 5. The originals have been enlarged about 
six-fold, and the portions reproduced are thus on a somewhat 
larger scale than Rowland's large map of the solar spectrum. 
Each of the strips reproduced is a little less than 20 Angstrom 
units in width, or considerably less than the distance between 
the series lines in the resonance spectra, the spacings of 
which vary from 36 to 38 units. The strong iron lines in 
this region appear on the plates for comparison. The 
absorption-lines are seen to be separated by distances which 
in some cases are less than 0*15 unit in width. When the 
resonance spectra have been photographed with the 12-foot 
grating, an immense amount of information can be obtained 
by comparing the spectrograms with this magnificent map 
of the absorption spectrum. This map will enable us to 
determine whether a given exciting line strikes an absorption- 
line exactly, or falls midway between two. It must be 
distinctly understood that the spectrum reproduced is a 
positive and not a negative of the absorption spectrum. It 
resembles a bright-line emission spectrum so closely, that I 
deem it of importance to draw attention to this fact. It is 
most remarkable that we can absorb portions of a continuous 
spectrum and leave regions not much over 0*05 or 0'1 of an 
Angstrom unit in width, and it would be interesting to 
examine this residual light, after transmission through the 
vapour with a Fabry and Perot interferometer. A discussion 
of this remarkable transmitted spectrum from the point of 
view of the pulse theory of white light should prove 
