812 



SCIENCE 



[N. S. Vol. XLI. No. 1066 



dense as air. A fair estimate of the total 

 light is that it is equal to 5,000 suns. 



These three stars, Algol, j3 Aurigce and 

 8 Ononis represent three types of eclipsing 

 binary. The first has a large faint com- 

 panion, in the second there are twin compo- 

 nents, while in the last case the bodies are 

 unequal in size but nearly equal in inten- 

 sity. As these were actually the first three 

 stars studied with the selenium photom- 

 eter, and something new came out of each, 

 it is evident that there is plenty of work to 

 be done on similar objects of which there 

 are thousands in the sky. There are at 

 least two other variables which we have 

 picked up, a Coronce Borealis, and the 

 bright star Spica. 



In fact the large proportion of stars 

 which are variable brings up a number of 

 questions. We may study a large number 

 of stars and find a certain number of 

 eclipsing variables. The proportion of vari- 

 ables gives the probability of such discov- 

 eries in a further search, but also we can 

 say that for every variable found there are 

 a definite number of other binary systems 

 the planes of whose orbits are inclined so 

 that we miss the eclipses altogether. From 

 considerations of this nature, it has been 

 possible to conclude: The preponderant 

 type of close binary with components of the 

 same order of size, and of equal or unequal 

 brightness, consists of bodies whose dis- 

 tance between centers is approximately 5 

 times their average radius, whose period of 

 revolution is about 4 days, and whose mean 

 density is 1/20 that of the sun. Systems of 

 greater or less relative separation are not 

 so numerous, or we should find more of 

 them among the eclipsing variables. This 

 particular discussion is based upon the vari- 

 ables which have been found by visual and 

 photographic methods, but there is abun- 

 dant field for work in the same line for the 

 electrical photometers. The point to em- 



phasize is that not only will systematic 

 studies of stars which vary in light give us 

 direct information, but indirectly we can 

 draw far reaching conclusions about stars 

 which are apparently constant. 



Of the many other problems in photom- 

 etry 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 meas- 

 ures of the general activity of the sun, 

 rather than of local disturbances like spots. 

 In direct measures 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 elimi- 

 nated 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 com- 

 parison 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 in- 

 stallation 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 



