24 ILLINOIS ACADEMY OF SCIENCE 



sodium, potassium, rubidium, or caesium. The sensitive 

 metallic surface is in an exhausted tube with a small quantity 

 of inert gas, and the effect of light is to release electrons, 

 which ionize the gas, and thus a current is produced. We are 

 fortunate in having several of our physicists at the Univer- 

 sity of Illinois interested in photo-electric cells, especially Pro- 

 fessor Jacob Kunz, and it is in the laboratory where the really 

 important improvements are made. When we produce a cell 

 m hich is twice as good as anything we have had before, this 

 amounts to the same things as though some good fairy had 

 suddenly doubled the light-gathering power of our telescope. 

 There are certain advantages of the photo-electric cell over 

 selenium, and while it is too soon to make a final estimate of 

 the relative sensibility, the newer device is already five or six 

 times as sensitive as the best we have ever had with selenium, 

 and we expect a still further improvement. 



The extreme sensibility required becomes apparent when we 

 state that the image of a second magnitude star, say the 

 Pole Star, near the focus of our twelve-inch telescope objective 

 gives the same surface illumination on a photo-electric cell 

 that would come from a candle at 500 meters' distance, with- 

 out any intervening lens. Therefore to measure the light of 

 such a star with a probable error of 1 per cent is equivalent 

 to the detection of a candle at 5,000 meters, or roughly three 

 miles. 



We may now consider some of the applications to the stars, 

 and although the results to be mentioned were all obtained 

 with the selenium photometer, they could have been secured 

 more easily with the photo-electric instrument if that had been 

 available. 



There is one star in the sky which for a hundred years 

 has aroused more interest than any other, namely, the well- 

 known variable, Algol. Once in sixty-nine hours the star 

 is found to lose two-thirds of its light, due to the eclipse of 

 the main body by a large and relatively faint companion. 

 This principal eclipse has been known and studied for a cen- 

 tury, but it has often been pointed out that if the eclipse theory 

 is true then, unless the companion is entirely dark, there should 

 be a second eclipse when it passes behind the main body. This 

 decrease in light midway between the primary eclipses was 



