THE LIGHT OF THE STARS. 47 



estimates 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 

 Almagest is No. 2,389 of the collection in the Bibliotheque Rationale 

 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 manu- 

 scripts 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 Persian astronomer, Abd-al-rahman al-sufi, who reobserved 

 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 mod- 

 ern 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, cover- 

 ing 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 required to com- 

 plete 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 catalogues are still unpublished.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 comparing 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 magnitudes for 2,785 

 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 valu- 

 able 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 modi- 

 fying 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 light of the stars, when only 

 the naked eye or a telescope is available, and much valuable work 

 might be done by applying it to the fainter stars, and especially to 

 clusters. 



