144 
ASTRONOMY: W. S. ADAMS 
portant of all, the hydrogen lines. In accordance with this system the 
stars are divided into seven main types designated by the letters B, A, 
F, G, K, M, and N, with intermediate types indicated on a scale ex- 
tending from zero to ten. Thus G5 indicates a type halfway between 
types G and K. The B stars are characterized by helium and hydrogen 
absorption lines. In the A stars the helium lines disappear, the hydro- 
gen lines reach their maximum intensity, and faint metallic lines begin 
to appear. These lines grow stronger and the hydrogen lines weaker in 
the successive t3^es F, G, and K, the low temperature lines in particu- 
lar increasing rapidly in intensity between the G and K types. The 
sun is a typical GO star. The M and N stars are distinguished by the 
presence of bands, in the one case of a compound of titanium, and in 
the other of carbon. 
The Harvard system of classification in general meets the require- 
ments of spectral observations in a most excellent way. There is, how- 
ever, in published descriptions of its application a serious lack of numeri- 
cal relationships between the intensities of the lines compared, and as 
a result a considerable uncertainty arises in the determination of spectral 
t)rpes. Since in many astronomical investigations a comparison is in- 
stituted between stars of very closely the same type it is important to 
reduce the classification of stellar spectra to as accurate a basis as pos- 
sible. The following brief description of the method employed at Mount 
Wilson is given for two purposes: first, because it replaces to a consid- 
erable extent direct estimations of spectral type by numerical estimates 
of relative line intensity which may be made with much higher accuracy; 
and second, because the method provides the material upon which sev- 
eral investigations have been based. It was devised in large measure 
by Dr. Kohlschiitter, and has been used with but slight modifications 
since his departure from Mount Wilson. 
The material available for classification purposes consists of several 
thousand photographs of stellar spectra taken with a one prism slit 
spectrograph and the sixty-inch reflector. About two- thirds of these 
spectra are of types succeeding FO. On most of the photographs the 
region of spectrum in best definition extends from X 4200 to X 4900. It 
includes, therefore the two hydrogen fines Hy and E^, the important 
calcium line at X 4227, and some of the most prominent iron lines in the 
entire spectrum. Since the hydrogen fines show a rapid decrease in 
intensity with the successive types F, G, K and M, and form by far the 
most important criterion in the derivation of spectral type, accurate de- 
terminations of their intensity relative to other lines in the spectrum are 
essential. Accordingly several adjacent iron lines have been selected 
