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XVII. 
ON A PHOTOGRAPHIC METHOD OF DETECTING THE 
EXISTENCE OF VARIABLE STARS. By J. JOLY, M.A.,. 
SoD, | 1s lain Se 
[Read Aprit 19; Received for publication Aprit 21; Published Junz 13, 1893.] 
Ir is very probable that the number of variable stars known to us. 
is but a small fraction of the actual number of such stars. Many 
of such stars doubtless possess so small a degree or so slow a. 
rate of variation of brightness as to render observation by us of 
the changes impossible, at least by any existing method of observa- 
tion. But there is probably a large number of stars, whose varia- 
tions might be observed if a continuous photometric record of their 
brightness could be kept. This method should be such as would 
keep under observation, not one, but groups of many stars simul- 
taneously, for there is, of course, a large degree of chance that 
an observer, systematically observing some few stars at random 
might not be so fortunate as to include a variable star among 
the number. 
Another quality which should be possessed by any photometric 
method applied to this study is that of being automatic, meaning 
by this that its record should be independent of the physiological 
state of the observer’s eyesight, and independently of him, con- 
tinuously record the intensity of illumination, for the variations 
sought for may be very slow. I venture to suggest the following 
method of meeting these requirements by the aid of photography. 
Ti the photographic plate, instead of being fixed within the 
telescope in the ordinary way, were driven with a slow eccentric 
circular motion, so that the image of each star describes a small 
circle on the surface of the plate, the intensity of the light from 
the star might be observed upon successive favourable nights, 
over long periods of time; the strength of the circular curve 
traced upon the plate depending upon the brightness of the star 
und the rate at which its image travels upon the sensitive film. 
