ASTRONOMY: F. H. SEARES 
237 
of the M's is peculiar, and owing to the lack of late K types the transition 
cannot certainly be traced. The interrelation of the B and A stars is also 
open to question. From the present results we should infer that for Ao stars 
there is little or no dependence of color upon absolute magnitude. This in- 
deed seems to be the case. For example, Vega and the companion to 
Eridani, both Ao and differing intrinsically by more than ten magnitudes, 
are very nearly of the same color. 
The change in the relation of color to luminosity from type to type is so 
rapid that a closer subdivision of the spectral classes is desirable, but this must 
await the accumulation of further data. 
Aside from the bearing of these results upon the problem of stellar constitu- 
tion, which it would be premature to consider at present, it is perhaps worthy 
9.4 
9.3 
9.2 
9.5 
9.4 
9.3 
9.6 
9.5 
• 
G6 
i 
• 
• 
-• 
• 
— • — * 
Ks 
T-* — 
M 
FIG. 3. VARIATION OF COLOR (LOGARITHM OF EXPOSURE-RATIO, AS ORDINATE WITH 
ABSOLUTE MAGNITUDE (ABSCISSA) FOR G, K, AND M STARS 
All the G and K stars have been reduced in color G6 and K5, respectively. Ma. Mb, 
and Mc stars have been combined without correction for color. 
of remark that they may occasionally be useful for determinations of distance 
Thus in the present case the results afford a useful control on the parallax of 
the Taurus Stream. As previously mentioned, it is only when reduced to zero 
absolute magnitude with the aid of the curve for the A5 stars that the colors 
of the Taurus stars are brought into agreement with the smooth curve for 
zero absolute magnitude defined by the adjacent spectral types (see fig. 1). 
Conversely, had the parallax of the Tautus Stream been unknown, it could 
have determined from the data now available with an indicated uncertainty 
of about 20 per cent, or dbO''005, which is comparable with the precision of the 
result usually adopted. 
The use of any such method, however, presupposes a knowledge of the spec- 
tral t3rpes of the stars in question. But if spectra are available, the spectro- 
scopic method of determining distance can immediately be employed; at least 
