DEVELOPMENTS IN ASTRONOMY PLASKETT. 263 



of star clusters and nebulae ever made have already been obtained 

 with the instrument, and its light efficiency in spectrographic work 

 is wonderful. It can obtain in five minutes a spectrum of a fifth 

 magnitude star that requires with our refractor over an hour. It is 

 no wonder that such an instrument excited the envy of all astrono- 

 mers who saw it, and Prof. Ritchey was pardonably proud of his 

 masterpiece. 



We turn from tins to, comparatively speaking, a rather insignificant 

 instrument, for measuring the brightness of the stars. The subject 

 of stellar photometry has always been a difficult one, as all the 

 photometers hitherto devised have depended upon eye estimates 

 or comparisons of the relative brightness of the star with either 

 another star or an artificial light, made by ingenious devices to 

 resemble and be brought close beside the star to be measured. 

 There is, in all such methods, the possibility of psychological errors, 

 and it has not been possible to obtain, except in special cases, results 

 with a lower probable error than about one-tenth of a magnitude. 

 In the case of the comparison of two stars brought into the one field 

 and equalized in intensity by polarizing apparatus, the probable 

 error is, perhaps, as low as three or four hundredths of a magnitude. 

 In another method also, in which out-of-focus images of the stars are 

 photographed, the density of the resulting disks have then to be 

 measured by a photometer and we have errors of the same order. 

 The new method, however, does not depend on eye estimates but on 

 the change in electrical resistance of the element selenium when 

 exposed to light. If a selenium cell is placed on the end of a telescope 

 and an image of a star to be measured thrown on it, the change of 

 resistance can be measured by a Wheatstone bridge arrangement and 

 very accurate values of the brightness obtained. Prof. Stebbins, 

 who has been working with much ability and energy on this problem 

 for the last three years, deserves much credit for his success in a 

 difficult research. He has recently made new measures of the light 

 curve of the well-known variable star Algol, and the probable error 

 of a determination at maximum is ± .006 mag., at minimum ± .023 

 mag. The accuracy of his observations enabled him to detect a 

 secondary minimum which had never before been seen and which 

 indicates that the companion whose eclipse of the bright star causes 

 the variability is not dark but light. Taking the most probable 

 value of the parallax or distance of the star, he finds that the bright 

 star, which has about the same diameter as the sun, gives 240 times 

 as much light, while the faint hemisphere of the companion gives 16 

 and the bright hemisphere 28 times the light of the sun. 



Such results as these are most interesting, and it is only by combi- 

 nation of several different methods, in this case of the light variation 

 by a photometer, the orbital elements by the spectroscope, and the 



