SYMPOSIUM ON ASTRONOMY 27 
only will systematic studies of stars which vary in light give 
us direct information, but indirectly we can draw far reach- 
ing conclusions about stars which are apparently constant. 
Of the many other problems in photometry which may be 
attacked with good prospect Of success may be mentioned the 
case Of our sun, which, according to Abbot, is a variable star. 
There can not be the slightest doubt of the variation, for a 
single sunspot is enough to change the total light, the only 
question is how much ? However, the changes in the light are 
probably measures of the general activity of the sun, rather 
than of local disturbances like spots. In direct measure of 
the sun's radiation the chief difficulty lies in the proper allow- 
ance for the absorption of the earth's atmosphere, but this 
trouble may be eliminated by comparing the reflected solar 
light from one of the planets with the light of a number Of 
stars. Probably Saturn is a good Object for this purpose, as 
there are few markings on its surface, but Uranus would be 
still better on account Of its slower motion, and the greater 
number of comparison stars which could be found for it. 
In the present paper, an attempt has been made to indicate 
in a general way the work we are doing, and evidently there is 
considerable variety in it. The production Of a good electric 
cell, and its proper installation in a photometer is a problem 
in experimental physics, and any success which has come has 
been through the efforts of several men of widely different 
training and interests. In the experiments with selenium I 
had the collaboration of Dr. F. C. Brown, and now, with photo- 
electric cells, Professor Jacob Kunz is doing his best to perfect 
our methods. By combining our knowledge and experience 
we have been able to carry on researches which would have 
been hopeless for one man alone. 
