ASTRONOMY: R. G. AITKEN 
531 
amined with the result that several hundred of our discoveries are fainter 
than this magnitude. It may develop that the most important con- 
tribution made by the survey, possibly the only one of permanent value, 
is this great addition to the number of known close visual binaries. 
Nevertheless, the statistical study has been the object which I have held 
definitely in mind from the beginning and I have regarded discovery as 
incidental. 
The present paper gives the results from a partial statistical study 
for the northern half of the sky, the data including all visual double 
stars (as distinguished from spectroscopic binaries) as bright as 9.0 B. 
D. magnitude which fall within the limits set by my Vorking definition' 
of a double star.^ In all there are 5400 such pairs north of the celestial 
equator, 2823 of which were discovered in the course of the Lick Ob- 
servatory survey. It is not claimed that this number is exhaustive, 
but it may fairly be said that it represents the capacity of the combi- 
nation of telescope and observer under average good observing conditions 
at Mount Hamilton. Unfortunately the data are not strictly homogene- 
ous, for part of the survey was carried out with the 12-inch refractor. 
A study of the relative efhciency of the 12-inch and 36-inch refractors 
in double star discovery leads to the conclusion that, if the entire work 
had been done with the latter telescope, about 250 additional pairs 
would have been discovered in the northern hemisphere. 
According to Seeliger's count, the Bonn Durchmusterung contains 
100,979 stars as bright as 9.0 magnitude north of the celestial equator; 
5400 of these, or one in 18.7 on the average, have actually been found 
to be double within the chosen limits. If we add only 200 more pairs 
the ratio is increased to 1 in 18.03. We may therefore conclude that 
at least one in every eighteen, on the average, of the stars as bright as 9.0 
magnitude in the northern half of the sky is a double star visible with the 
36 -inch telescope. 
Table I shows the distribution of these double stars in galactic latitude 
by magnitude classes, the arguments, for convenience of comparison, 
being those adopted by Seeliger^ in his discussion of the B. D. stars. 
The zones are 20 wide, the first extending from the north galactic pole 
to H-70°, the fifth from -flO to -10 galactic latitude. Zone IX, 
touching the south galactic pole, lies entirely south of the celestial 
equator. The progressive increase in numbers in every magnitude 
class as we approach Zone V from the north or from the south was to 
be expected, for it is well known that the stars as a whole show a similar 
distribution. When, however, we compare the results with Seeliger's 
tabulation for all the B. D. stars to 9.0, we find that the frequency curves 
