MAGNITUDES OF STARS. 121 



49 stars distributed almost equally over both hemispheres. 

 Considering that the catalogue enumerates about one-fifth 

 of all the fixed stars visible to the naked eye, it should, 

 according to Argelander's investigations, have given 640 stars 

 of the 6th magnitude. The nebulous stars (i/e<eXoei8ets) of 

 Ptolemy and of the Pseudo-Eratosthenian Catasterisms, are 

 mostly small stellar swarms,"* appearing like nebulae in the 

 clearer atmosphere of the southern hemisphere. I more 

 particularly base this conjecture on the mention of a nebula in 

 the right hand of Perseus. Galileo, who, like the Greek and 

 Arabian astronomers, was unacquainted with the nebula in 

 Andromeda which is visible to the naked eye, says in his 

 Nunclus sidereus, that stellce nebulosce are nothing more than 

 stellar masses scattered in shining groups through the ether 

 (areolce sparsim per cethera fulgent}** The expression (r&v 

 fieyd\a>v rots), the order of magnitudes, although referring only 

 to lustre, led, as early as the ninth century, to hypotheses on the 

 diameters of stars of different brightness : ** as if the intensity 

 of light did not depend on the distance, volume, and mass, 

 as also on the peculiar character of the surface of a cosmical 

 body in. more or less favouring the process of light. 



At the period of the Mongolian supremacy, when, in the 

 fifteenth century, astronomy flourished at Samarcand, undei 

 Tiniur Ulugh Beig, photometric determinations were facilitated 

 by the subdivision of each of the six classes of Hipparchus 

 and Ptolemy into three subordinate groups ; distinctions, for 

 example, being drawn between the small, intermediate, and 



62 Ptol. Almag. ed. Halma, torn. ii. p. 40, and in Eratosth. 

 Catast., cap. 22, p. 18. q &' KetyaXr) KCU 17 apm} avcnrros opera;, 

 8ta 8e V(frf\<a8ovs (rva"Tpo(f)rjs 8oKfl TKTLV opdadai. Thus, tOJ 

 Geminus, Phcen. (ed. Hilder, 1590), p. 46. 



63 Cosmos, vol. ii. pp. 71314. 



64 Muhamedis Alfragani Chronologica et Ast. Elementa, 

 1590, 3ap. xxiv. p. 118. 



