80 SPECIAL RESULTS IN THE UEA.KOLOGICAL 



the classification of the Almagest at the close of the Table of 

 Stars in the 8th book. Ptolemy, with reference to unas- 

 sisted vision, called all stars " dark" which were fainter than 

 his 8th class, of which class singularly enough he gives 

 only 49, almost equally distributed in the two hemispheres. 

 Considering that the table includes about a fifth part of the 

 stars visible to the naked eye, it ought, according to Arge- 

 lander's investigations, to have given 640 stars of the 6th 

 magnitude. The nebulous stars (veQeXoethlQ) of Ptolemy, 

 and of the Catasterisms of the Pseudo-Erastosthenes, are 

 mostly small clusters of stars ( 151 ) which, in the purer air of 

 southern skies, appear as nebulae. I rest this conjecture 

 more particularly on the mention of a nebula in the right 

 hand of Perseus. Galileo, to whom, as well as to the Greek and 

 Arabian astronomers, the nebula in Andromeda, although 

 visible to the naked eye, was unknown, said, in the Nuncius 

 sidereus, that " stellse nebulosse" are no other than clusters 

 of stars, which as " areola? sparsim per sethera fulgent." ( 152 ) 

 The expression "order of magnitude" (T&V /ueyaXwv rafe), 

 although referring only to brilliancy, yet led, as early 

 as the 9th century, to hypotheses respecting the diameters of 

 stars of different degrees of brightness ; ( 153 ) as if the in- 

 tensity of the light did not depend on the distance, the 

 volume, the mass, and the peculiar nature or character of 

 the surface of the body, as more or less favourable to the 

 luminous process or production of light. 



At the time of the Mogul Power in the 15th century, 

 when, under Ulugh Beig, the descendant of Timour, 

 astronomy flourished in the highest degree at Samarcand, 

 photometric determinations received such an impulse, that 

 each of the six classes or orders of magnitude of Hipparchus and 



