Measuring the Light of the Stars 



By Joel Stebbins 



Professor of Astronomy in the University of Illinois 



Prof. Stebbins' remarkable measurements of the heat of stars have attracted 

 the attentio7i of astronomers all over the world. Apart from the value of the results 

 obtained, his work is interesting because it shows that astronomers are making use 

 of modern technical advances, as in the case which he describes, sometimes before 

 they are perfected for commercial purposes. — ■Editor. 



ONE of the standard problems of 

 astronomy is the exact determina- 

 tion of the amount of light that 

 comes from each of the stars. Not that 

 the knowledge of the fraction of a candle 

 power of each star is of any interest or 

 importance, but that the measures are 

 valuable for future reference, especially 

 to determine the gradual changes in 

 light caused by the dying out or the 

 brightening of these distant objects. 

 Our own sun being one of a class of stars, 

 the best clue to the life history of the sun 

 may be given by a study of other bodies 

 of the same kind. We also find in the sky 

 numerous extraordinary objects, called 

 short-period variable stars, which change 

 in brightness by fifty per cent or more in 

 the course of a few days, or even hours. 

 Wanted: A Standard Eye 

 For general purposes the unaided 

 human eye is one of the best instruments 

 fpr measuring the light of stars, and most 

 f.orms of photometer depend ultimately 

 upon the eye for a comparison of two 

 lights. Because of the difference between 

 individuals, however, there is no such 

 thing as a "standard eye," and astrono- 

 mers have long been waiting for some 

 purely mechanical device which will 

 register light intensities. Let us note 

 that such an instrument is even more in 

 demand for commercial work, especially 

 for testing electric lights. At present the 

 ordinary householder has to take the 

 word of somebody else for the amount of 

 light he is getting from electric lamps. 

 The lighting companies accommodate us 

 with meters telling how much current we 

 use, but we have no exact measure of how 

 much light they are delivering. City 

 authorities contract for a number of 

 lamps of say one thousand candle power 

 each, but who knows after the lamps are 

 installed whether they furnish a thousand 



or only eight hundred candle power? 



We see that there is a real demand for 

 an instrument which, held at a given 

 distance from any lamp, will indicate 

 just how much light is being emitted. 

 Needless to say, many experimenters 

 have attempted to perfect such an instru- 

 ment, but so far without success. The 

 underlying principle of these devices has 

 been to make use of some substance 

 which changes its properties under the 

 influence of light. One of the most im- 

 portant is the element selenium, a sub- 

 stance in the same chemical group as 

 sulphur. For more than a generation it 

 has been known that the crystalline form 

 of selenium changes its electrical resist- 

 ance when exposed to light. Other sub- 

 stances exhibit this same property, but 

 none to such a marked degree as selen- 

 ium. The ordinary arrangement is called 

 a cell or bridge. Two wires are wrapped 

 about an insulator, and on one face the 

 selenium is deposited and then sensi- 

 tized. The best method of sensitizing is 

 a trade secret, but one standard method 

 is to melt the selenium at four hundred 

 and twenty degrees Fahrenheit, and 

 then let it cool gradually, when it will 

 crystallize and be light-sensitive. There 

 must be a certain amount of mystery 

 in the process, even to the makers them- 

 selves, for none of them can furnish cells 

 of a standard resistance, nor even two 

 cells which are precisely alike. On the 

 opposite page is shown an unmounted cell 

 of the usual form. In the dark it has an 

 electrical resistance of about five hundred 

 thousand ohms, but on exposure to strong 

 daylight the resistance drops to about 

 ten thousand ohms, or only one-fiftieth 

 of the original. 



The principle of a selenium photom- 

 eter is, then, to connect a selenium cell 

 with a small battery and to measure the 



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