122 COSMOS. 



larye stars of the se 3ond magnitude an attempt which reminds 

 us of the decimal gradations of Struve and Argelander. 66 

 This advance in photometry, by a more exact determination 

 of degrees of intensity, is ascribed in Ulugh Beig's tables to 

 Abdurrahman Sufi, who wrote a work " on the knowledge of 

 the fixed stars," and was the first who mentions one of the 

 Magellanic clouds under the name of the White Ox. Since 

 the discovery and gradual improvement of telescopic vision, 

 these estimates of the gradations of light have been extended 

 far below the sixth class. The desire of comparing the in- 

 crease and decrease of light in the newly appeared stars in 

 Cygnus and Ophiuchus (the former of which continued 

 luminous for twenty-one years), with the brightness of other 

 stars, called attention to photometric determinations. The 

 so-called dark stars of Ptolemy, which were below the 6th 

 magnitude, received numerical designations according to the 

 relative intensity of their light. ' Magnitudes, from the 8th 

 down to the 16th," says Sir John Herschel, "are familiar to 

 those who are in the practice of using powerful instruments." 66 

 But at this faint degree of brightness, the denominations for 

 the different gradations in the scale of magnitudes are very 

 undetermined, for Struve occasionally classes, among the 12th 

 or 13th, stars which Sir John Herschel designates as belonging 

 to the 18th or 20th magnitudes. 



The present is not a fitting place to discuss the merits 

 of the very different methods which have been adopted for the 

 measurement of light within the last hundred-and-fifty years, 

 from Auzout and Huygens to Bouguer and Lambert ; and 

 from Sir William Herschel, Rumford, and Wollaston, to 



66 Some MSS. of the Almagest refer to such subdivisions 

 or intermediate classes, as they add the words fieifaw or eXao-cra>z> 

 to the determination of magnitudes. (Cod. Paris, no. 2389.) 

 Tycho expressed this increase or diminution by points. 



66 Sir John Herschel, Outlines of Ast~., pp. 520-27. 



