PROCEEDINGS OP THE AMERICAN ACADEMY 



Investigations on Light and Heat, pobltshed with an appropeiation from the 

 RuMFORD Fund. 



XXL 



PHOTOMETRIC MEASUREMENTS OF THE VARIABLE 

 STARS ^ PERSEI AND DM. 81°2o, MADE AT THE 

 HARVARD COLLEGE OBSERVATORY. 



By Edward C. Pickering, Director, Arthur Searle and 

 0. C. Wendell, Assistants. 



Presented April 13, 188L 



Our knowledge of the cause of variation in the light of certain of 

 the fixed stars must be derived largely from the curves showing the 

 intensity of their light at any given time. Two methods may be em- 

 ployed for determining the form of these light-curves, as they are 

 called. First, that proposed by Argelander, in which the variable is 

 compared by the eye with some adjacent stars of nearly equal bright- 

 ness. The difference, if any, is estimated in terms of a small unit called 

 a grade, which nearly equals a tenth of a magnitude. A discussion 

 of the entire series of measures serves to determine the light of the 

 comparison stars, and to reduce all the measures to a scale of grades. 

 Tliis method is so simple, and gives results of such precision, that it has 

 heretofore been almost exclusively used. For determining the form 

 of the curve qualitatively, and the times of maximum and minimum 

 light, this method leaves little to be desired. For a quantitative study 

 of these curves, however, we must reduce the scale of grades to light 

 ratios by photometric measures of the comparison stars. If, mean- 

 while, any of the comparison stars vary in light, errors are introduced 

 which cannot be eliminated, and these, with the errors in the photometric 

 measures, are likely to greatly exceed the errors in determining the form 

 of the light-curve. The second method consists in a photometric meas- 

 urement of the light of the variable at different times, and thus deter- 

 mining directly the form of its light-curve. Although the errors in 

 the final results in the second metliod may be no larger than in the first, 

 yet they are rendered much more conspicuous, so that hitherto no 

 very satisfactory light-curves have been obtained in this way. On the 



