THE LIGHT OF THE STARS 375 



ing for /-let'Cw (nieizon), large or bright, if the star was faint he added 

 e (epsilon), standing for e'Aao-o-wv (elasson), small or faint. These esti- 

 mates were presumably carefully made, and if we had them now, they 

 would be of the greatest value in determining the secular changes, if 

 any, in the light of the stars. The earliest copy we have of the Alma- 

 gest is no. 2389 of the collection in the Bibliotheque Nationale of 

 Paris. It is a beautiful manuscript, written in the uncial characters 

 of the ninth century. A few years ago it could be seen by any one in 

 one of the show-cases of the library. There are many later manuscripts 

 and printed editions which have been compared by various students. 

 The errors in these various copies are so numerous that there is an 

 uncertainty in the position, magnitude, or identification of about two 

 thirds of the stars. A most important revision was made by the Per- 

 sian astronomer, Abd-al-rahman al-Sufi, who re-observed Ptolemy's 

 stars, A. D. 964, and noted the cases in which he found a difference. 

 The careful study and translation of this work from Arabic into 

 French by Schjellerup has rendered it readily accessible to modern 

 readers. 



No important addition to our knowledge of the light of the stars 

 was made until the time of Sir William Herschel, the greatest of 

 modern observers. He found that when two stars were nearly equal, 

 the difference could be estimated very accurately. He designated 

 these intervals by points of punctuation, a period denoting equality, 

 a comma a very small interval, and a dash a larger interval. In 1796 

 to 1799, he published in the Philosophical Transactions four catalogues 

 covering two thirds of the portion of the sky visible in England. 

 Nearly a century later, it was my great good fortune, when visiting his 

 grandson, to discover in the family library the two catalogues re- 

 quired to complete this work, and which had not been known to exist. 

 These two catalogues are still unpublished. Meanwhile, little or no use 

 had been made of the four published catalogues which, while com- 

 paring one star with another, furnished no means of reducing all to 

 one system of magnitudes. The Harvard measures permitted me to 

 do this for all six catalogues, and thus enabled me to publish magni- 

 tudes for 2785 stars observed a century ago. with an accuracy nearly 

 comparable with the best work of the present time. For nearly half 

 a century no great advance was made, and no astronomer was wise 

 enough to see how valuable a work he could do by merely repeating 

 the observations of Herschel. Had this work been extended to the 

 southern stars, and repeated every ten years, our knowledge of the 

 constancy of the light of the stars would have been greatly increased. 

 In 1844, Argelander proposed, in studying variable stars, to estimate 

 small intervals, modifying the method of Herschel by using numbers 

 instead of points of punctuation, and thus developed the method 

 known by his name. This is now the best method of determining the 



