PORTION OF THE COSMOS. PHOTOMETRY. 81 



Ptolemy were divided into three subdivisions; distinguishing, 

 for example, small, middling, and large, of the second mag- 

 nitude ; reminding us of the attempts of Struve and 

 Argelander, in our own day, to introduce decimal grada- 

 tions^ 154 ) In the tables of Ulugh Beig, this photometric 

 advance, or more exact determination of different degrees of 

 light, is ascribed to Abdurrahman Sufi, who had published a 

 work specially " on the knowledge of the fixed," or fixed 

 stars, and who first noticed the existence of one of the ma- 

 gellanic clouds under the name of the " White Ox." Since 

 the introduction of telescopic vision, and its gradual improve- 

 ment, estimations of successive gradations of light have ex- 

 tended far beyond six classes or magnitudes. The desire of 

 comparing with the light of other stars the newly-appeared 

 stars in Cygnus and Ophiuchus, (the first of which continued 

 to shine for 21 years), at successive stages of their increasing 

 and decreasing light, gave a stimulus to photometric con- 

 siderations. The so-called " dark" stars of Ptolemy, or those 

 below the 6th magnitude, received numerical denominations 

 corresponding to the relative intensity of their light. " As- 

 tronomers," says Sir John Herschel, "who are accustomed 

 to the use of powerful space-penetrating telescopes, pursue the 

 descending gradations of light from the 8th down to the 16th 

 magnitude." ( 155 ) But with such faint degrees of light, the 

 denominations of the different classes of magnitude some- 

 times become very uncertain; and Struve occasionally 

 reckons as belonging to the 12th or 13th magnitude stars 

 which John Herschel calls of the 18th or even 20th. 



This is not the place for examining in a critical manner 

 the very different methods which have been applied to 

 photometric determinations, during the last century and a half, 



VOL. III. G 



