PROCEEDINGS 
OF THE 
NATIONAL ACADEMY OF SCIENCES 
Volume 1 MAY 15. 1915 Number 5 
^^^^^^ 
SOME PROBLEMS IN STELLAR PH0T0M£TRMAV Sf 
By Joel Stebbins 
ASTRONOMICAL OBSERVATORY. UNIVERSITY OF ILLINOIS 
Read before the Academy. April 20, 1915. Received, April 1,1915 
It is the purpose of this note to point out some of the researches 
which have been made possible by the development of the selenium and 
photo-electric photometers. The first advantage of these electrical de- 
vices is a considerable increase in accuracy over what is possible by vis- 
ual and photographic methods. Perhaps the best field for immediate 
investigation lies in the study of the eclipsing variables, which give us 
the only direct clue to the actual diameters of stars. Nearly one hun- 
dred of these systems are now known, and new ones are continually 
being found. In the various studies which have been made, there is 
one line of reasoning which seems to have been overlooked, as follows. 
Among the systems discovered, some are constant in light for a large 
fraction of the period, while others have relatively long times of varia- 
tion, and are thus easier of discovery and observation. If P be the total 
period, and D the time of duration of the eclipse, the probability that 
a star will be faint when observed at any time is D/P. It seems plausi- 
ble to assume that if we take the number of discovered stars of each 
class and multiply by P/D we shall get numbers representing the rela- 
tive occurrence of such objects which are actually visible and might be 
discovered. For example: suppose that in a random search about the 
sky we find 5 stars where the eclipse is i, and 5 where the ecHpse is ro- 
of the period. Then instead of the relative numbers awaiting discovery 
being equal, it seems much more probable that there are about 25 of 
the first class to 50 of the second. 
There is another consideration that for every double system which 
exhibits eclipses in our direction, there are others whose orbit planes are 
inclined so that we miss the eclipse altogether. We may adopt half a 
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