30 
ASTRONOMY: F. H. SE4RES 
which is a function of the star's intrinsic luminosity and also the influ- 
ence of a possible scattering of light in its passage through space. 
The following paragraphs indicate briefly the results found with the 
60-inch reflector for about 80 of the North Polar Standards between 
magnitudes 2.5 and 16.3 whose colors had already been derived by a 
comparison of their photographic and photovisual magnitudes. The 
brighter stars, owing to their distribution, had to be observed sepa- 
rately. To avoid photographic difficulties, the plates for these were 
all of the same emulsion. Further, all the brighter objects were reduced 
to an equivalent of the 11 th magnitude, approximately, by means of 
screens and diaphragms and then given the following series of exposures 
on a Cramer Inst. Isochromatic plate: 
a. Yellow images (through yellow filter), 16% 32^ 
b. Blue images (without filter), 2% 4% 8% 16% 32^ 
c. Yellow images (through yellow filter), 16% 32«. 
In a few cases the exposures for the yellow images were 32^ and 64^ 
The data on each plate were reduced graphically and gave two values 
for the exposure-ratio corresponding to equal blue and yellow images. 
In general, three plates, exposed upon different nights, were used for 
each star. To derive a reduction curve connecting color-index with 
exposure-ratio, the mean ratios were collected in groups according to 
color-index. Since each group contains stars of widely differing magni- 
tudes, the resulting curve is probably free from the influence of any 
systematic error in the color-indices depending on brightness, and should 
give a reliable value for the color of any star whose exposure-ratio has 
been determined under conditions similar to those underlying the deriva- 
tion of the curve itself. Moreover, the question of systematic errors 
in the original color-indices, which were taken from the Mount Wilson 
investigation of the photographic and photovisual magnitudes of the 
polar stars, can be put to a direct test. 
For this purpose the color-index of each star was determined from its 
exposure-ratio by means of the reduction curve and compared with the 
original value based on magnitudes. The differences, arranged accord- 
ing to the brightness of the stars, should reveal any systematic error 
in the original system of color values, and thus test the accuracy, rela- 
tive to each other, of the photographic and photovisual scales of the 
North Polar Standards. The means of these differences for groups of 
stars of a limited range of brightness are given in the second column of 
the table. Obviously there are no systematic differences of any impor- 
tance which depend upon magnitude. 
In order that the investigation might be extended to the fainter stars. 
