SYMPOSIUM ON ASTRONOMY 23 



STELLAR PHOTOMETRY 

 By Joel Stebbins 



In measures of the light of stars we are not concerned with 

 absolute intensities, but rather with how the light of a heavenly 

 body varies. If the light is constant, there is not much to be 

 learned, but if it changes, we may infer a great deal from the 

 law of variation. In laboratory and commercial photometry, it 

 i^ customary to measure what may be called the visual bright- 

 ness of a source of light, but with the stars it is immaterial 

 for many purposes whether we study the changes of the red, 

 or the blue, or any other part of the spectrum, though in fact 

 any complete stellar photometry should include measures in 

 all regions, infra-red, visible, and ultra-violet. 



The chief disadvantage in stellar photometry is that the 

 stars are so faint that it is usually not feasible to expand 

 their images out into surfaces, and most forms of stellar photo- 

 meter depend upon comparisons of two point images by the 

 eye. Although the eye is a wonderful instrument, especially 

 in the range of intensity over which it may be used, the limit 

 of accuracy attained by looking first at one light and then at 

 another is much the same as though instead of using a bal- 

 ance we should weigh objects by lifting them in our hands. It 

 is safe to say that no observer has been able to get visual re- 

 sults accurate to 1 per cent, and in the best measures there are 

 occasional errors of 10 per cent, 20 per cent, and even more. 

 It was hoped that the introduction of photography would bring 

 greater accuracy in stellar photometry, but at present the er- 

 rors of the best photographic measures and of the best visual 

 ones are about the same. 



For a number of years we have been interested at our ob- 

 servatory in the development of an electrical method for the 

 measurement of star light, based upon the property of the 

 peculiar substance selenium. There is another device, how- 

 ever, which bids fair to supplant entirely the selenium photo- 

 meter, namely the photo-electric cell made from one of the 

 alkali metals. The principle of each of these devices is the 

 conversion of a light effect over into a minute electric cur- 

 rent which can be measured by a galvanometer or electro- 

 meter. In the photo-electric cells we use one of the metals. 



