PORTION OP THE COSMOS. PHOTOMETRY. 83 



Argo, in 1837, have made the want of better photometric 

 processes more than ever felt. 



It is material to distinguish between the mere successive 

 arrangement of stars in the order of their brilliancy, but with- 

 out numerical estimations of the intensity of light, (the 

 Scientific Manual for Naval Officers, published by the British 

 Admiralty, contains such a list), and classifications with num- 

 bers appended expressing the intensity of light, either under 

 the form of so-called relations of magnitude, or by the more 

 hazardous assignment of the quantities of radiated light. ( 158 ) 

 The first numerical series, founded on estimations with the 

 naked eye, but progressively improved by a careful revision 

 of the materials, ( 159 ) probably deserves, in the present very 

 imperfect stateof photometric apparatus, the preference among 

 the different approximate methods ; although the exactness 

 of the estimations is no doubt impaired by differences in the 

 individual powers and habits of different observers, the 

 clearness of the atmosphere, the different altitudes of the 

 stars which are to be compared, and which can only be so 

 by means of many intermediate links, and, above all, by 

 inequalities of colour. Yery bright stars of the 1st magni- 

 tude, Sirius and Canopus, a Centauri and Achernar, Deneb 

 and a Lyrse, though they have all white light, are much 

 more difficult to compare with each other by estimation with 

 the naked eye, than are stars of fainter light, as, for ex- 

 ample, those below the 6th and 7th magnitudes. But the 

 difficulty is still greater with stars of very intense light, 

 when yellow stars like Procyon, Capella, or Atair, are to 

 be compared with red ones like Aldebaran, Arcturus, and 

 Betelgeuze.( 160 ) 



Sir John Herschel, by means of a photometric comparison 



