ASTRONOMY: W. S. ADAMS 
147 
This simple method of classification may be recommended as being 
rapid of operation, and free from the difficulties connected with the com- 
parison of separate photographs with one another. It requires the es- 
tablishment of a scale of relative-intensity-estimates by the observer, 
but this is a very simple matter when the range employed is small. To 
some extent the scale will be dependent upon the dispersion of the spec- 
trograph employed since several of the lines used are compound in 
character. With the single prism spectrograph at Mount Wilson the 
same reduction curves have been used successfully for photographs on 
which the linear dispersion varies from 16 to 90 angstrom units to the 
millimeter at the center of the spectrum. 
In connection with the classification of stellar spectra a number of 
photographs have been made with a Koch microphotometer of the in- 
tensity curves of some of the pairs of lines employed in the comparison. 
There are numerous practical difficulties connected with the use of this 
instrument for lines as narrow and as short as those in stellar spectra, 
and it is doubtful whether the accuracy obtained is of so high an order 
as to justify the use of so laborious a method for stellar classification. 
It is probable, however, that it might be used to advantage in the se- 
lection of standard stars of reference in which a knowledge of the abso- 
lute intensities of certain spectrum lines would be of great value. 
INVESTIGATIONS IN STELLAR SPECTROSCOPY. II. A SPEC 
TROSCOPIC METHOD OF DETERMINING STELLAR 
PARALLAXES 
By Walter S. Adams 
MOUNT WILSON SOLAR OBSERVATORY. CARNEGIE INSTITUTION OF WASHINGTON 
Recdved by the Academy, Febraary 8, 1916 
The question whether the intrinsic brightness of a star may not have 
an appreciable effect upon its spectrum is one with important appHca- 
tions in astronomy. If two stars which have closely the same type of 
spectrum differ very greatly in luminosity it is probable that they also 
differ greatly in size, mass, and in the depth of the atmospheres sur- 
rounding them Accordingly we might hope to find in these stars cer- 
tain variations in the intensity and character of such spectrum lines as 
are peculiarly sensitive to the physical conditions of the gases in which 
they find their origin, in spite of the close correspondence of the two 
spectra in general. If such variations exist and a relationship may be 
derived between the intensities of these fines and the intrinsic brightness 
