J. C. Draper—Dark Lines in the Solar Spectrum. 259 
iron and oxygen 8 it 
protearaphs, by projection under a suitable magnifying power. 
he center of each line was the portion from whic measures 
were taken in all cases. The wave lengths of the oxygen lines 
were then determined by means of a curve, which from 43864 
to 4441475 was based on the iron lines of the same spectrum. 
In all forty-seven iron lines in this extent of the spectrum, or 
about one to every eleven wave lengths were used. The 
values assigned to the iron lines were those obtained in my 
chart of the solar spectrum. By this method of measurement, 
errors arising from maladjustment of two spectra were avoided. 
From 44414°75 to 44705, the iron lines did not photograph, L 
was therefore obliged to construct this portion of the curve 
from the wave lengths of oxygen and air lines already given 
by various authorities ; selecting those values in which they 
agreed. From 8864 to 441475 the results are therefore accu- 
tate. From 4414-75 to 4705, though they are approximate, the 
error if any exists, must be very small. The measurements to 
fractions of a wave length were obtained by constructing the 
are found 
line is the scale of wave lengths, each wave length being five 
Mn 
* Ree 1 3 
millimeters in extent. In the second space the lines of Ang- 
Strom’s chart are given. In the third space the lines measurec 
on the photographs, the vertical portion of each symbol 
