258 J.C. Draper—Dark Lines in the Solar Spectrum. 
gated, and their action on the details of the picture determined. 
Pictures may even be placed face to face, one a little above 
the other, and examined in that position by projection. From 
the measures thus obtained a chart of the spectrum was con- 
structed, which extended from E in the green to P in the 
ultra-violet. The values assigned to the wave lengths in this 
chart are those of Angstrém, and it is my purpose to present 
the positions and characters of certain of these lines in this 
communication. 
The great increase in the number of lines in the chart made 
from photographs by Mr. Rutherfurd’s grating, compared with 
that of Angstrém led me to collect all the measurements of 
spectrum lines of elements that I could find, for the purpose of 
determining the character of the newly measured lines. On 
comparing the lines of the spectra of oxygen, nitrogen and air, 
as given in Watt’s index of spectra, from the researches of 
Thalén, Huggins and Pliicker, ] was struck with the number 
amounting to half a wave length could therefore exist in the 
position of a line, according as it fell on one side or the other 
of a figure on the scale expressing a wave length. In the 
values given to the air lines by Angstrém in his chart, this 
difficulty did not exist ; I therefore attempted the use of Ang- 
strém’s values, employing the work of Huggins and Pliicker, 
to separate as far as possible the oxygen from the nitrogen 
lines. This operation was, however, quickly discarded ; because 
of the great differences existing between these authorities re- 
rding the wave lengths of a number of oxygen and air lines. 
o obviate this trouble, I made photographic measurements of 
the lines of the electric spectrum of oxygen by the method 
detailed below. 
The apparatus employed consisted of a spectroscope with 
two flint glass prisms of 60°, adjusted to the minimum devia- 
tion of D’. Collimator and telescope objectives, achromaties of 
ten inches focus. This was used to make photographs of the 
spectra given by the condensed electric spark in oxygen, in alr 
and in nitrogen. When so employed the eye-piece of the 
telescope was removed, and a camera placed in its stead. The 
slit was sometimes made as narrow as was possible. The 
induction coil was one of Ritchie’s, iving a ten-inch spark, and 
having a hammer current-breaker driven by clock work. The 
battery was three two-gallon bichromate cells, the elements 
