150 
ASTRONOMY: W. S. ADAMS 
sun-spots, and increases in intensity with reduction in temperature. 
The Hne X 4216 of strontium, on the other hand, is an enhanced Hne, 
that is stronger in the spectrum of the spark than of the arc, and is prob- 
ably a high temperature Hne. It is very prominent in the spectrum of 
the sun's Hmb when photographed at ecHpses, and also in the upper 
chromosphere. Numerous other smaller differences between the spec- 
tra of OL Tauri and 61^ Cygni all point in the same direction; the low tem- 
perature lines strengthened in sun-spots are stronger in 61^ Cygni; the 
enhanced lines are stronger in a Tauri. 
It has seemed preferable, however, for two reasons to use only these 
two lines in the absolute magnitude investigation. First, because they 
show the effect most markedly; and second, because they appear to be 
influenced but slightly by closely adjoining lines which blend with them. 
Among other lines which show the effect plainly, reference should be 
made to X 4435 of calcium and X 4535 of titanium, which are strong in 
intrinsically faint stars, and to two lines at X 4395 and X 4408 which are 
strong in the brighter stars. The Hne at X 4395 is probably due to en- 
hanced titanium. As will appear later, in the course of a discussion of 
M type stars, the hydrogen lines themselves seem to vary with absolute 
magnitude, at least in certain types of spectra. This should prove of 
fundamental importance in further investigations of stellar luminosity. 
After the behavior of the two lines X 4216 and X 4455 had been exam- 
ined in a large number of stars, and the systematic differences had been 
found to persist through a wide range of spectral type, the attempt was 
made to estabHsh a numerical relationship between the intensities of 
these lines and the absolute magnitudes of the stars in which they occur. 
As in the case of the hydrogen lines used for classification purposes, 
Hnes were selected near X 4216 and X 4455, with which the intensities of 
these lines were compared, the differences of intensity being estimated 
on a scale extending from zero to ten. The pairs of lines finally adopted 
for all of this work are as follows: 
(a) X 4216, Sr and X 4250, Fe 
(b) X 4455, Ca X 4462, Fe, 
(c) X 4455, Ca X 4495, Fe 
For convenience of reference these pairs of lines wiH be designated in 
the future as (a), (b) and (c). The value (a) = —2, for example, denotes 
that X 4216 is estimated to be two units fainter than X 4250. 
As soon as the estimates had been completed a number of the stars 
with well-determined paraHaxes were selected, their absolute magnitudes 
were computed, and curves were constructed in which the observed dif- 
ferences of intensity for each pair of lines formed the abscissae, and the 
