ASTRONOMY: H. SHAPLEY 
525 
EXPOSURE-RATIOS 
PLATE 
DATE 
HOUR ANGLE 
Blue/Yellow 
Blue/Violet 
P 3138 
1916, June 25 
25%V 
0.168 
0.496 
315U 
zo 
lili 
U . ioO 
3163 
27 
27E 
U . i / o 
yj . J JO 
3167 
27 
6E 
n 1 80 
\} . lOU 
U . Dyj 
3171 
27 
15W 
\J . LOO 
3173 
27 
37W 
0.164 
0.533 
3177 
27 
58W 
0.172 
0.547 
Mean 
0.168 
0.553 
Average deviation from mean .... 
=t 0.007 
±0.027 
Change in ratio for 0.1 spectrum interval 
0.004 
0.014 
Spectrum interval corresponding to average devia- 
=t 0.180 
=b0.190 
From the accordance of results for the same star, derived mainly 
upon different nights, it is found that the average deviation in the 
exposure-ratio from a single plate corresponds to about 0.2 of a spectrum 
interval, or 0.08 mag. in the color-index. The greater redness of stars 
of high luminosity, spectral type remaining the same, is easily seen in 
the results from even a single plate. The precision seems to be the same 
whether the yellow or the Adolet is used for comparison with the blue. 
As none of the results have yet been corrected for atmospheric extinc- 
tion, the average deviation includes the effect of this disturbing factor. 
For illustration the results for seven plates on one of the stars, 72 w 
Herculis, are given in the table. 
1 Photovisual magnitudes express the visual brightness of a star, but are determined 
photographically. The procedure is the same as for the determination of photographic 
magnitudes, except that an isochromatic plate combined with a yellow filter is used instead 
of the ordinary photographic plate. 
2 See for example Hertzsprung, Mi. Wilson Conlr., No. 100; Astroph. /., 42, 92 (1915). 
3 These Proceedings, 1, 481 (1915). 
< ML Wilson Contr., No. 80, 19; Astroph. /., 39, 325 (1914). 
STUDIES OF MAGNITUDES IN STAR CLUSTERS. III. THE 
COLORS OF THE BRIGHTER STARS IN FOUR 
GLOBULAR SYSTEMS 
By Harlow Shapley 
MOUNT Wll^ON SOLAR OBSERVATORY. CARNEGIE INSTITUTION OF WASHINGTON 
Received by the Academy, August 3. 1916 
In a former communication dealing with the probable sequence of 
spectral types in stellar development^ it was shown that the brightest 
stars in the great Hercules cluster are of the redder spectral types, while 
