60 COSMOS. 



cient to mention the names of Airy, Arago, Biot, Brewster, 

 Cauchy, Faraday, Fresnel, John Herschel, Lloyd, Mains, 



Neumann, Plateau, Seebeck, to remind the scientific 



reader of a succession of splendid discoveries, and of their 

 happy applications. The great and intellectual labours of 

 Thomas Young more than prepared the way for these im- 

 portant efforts. Arago' s polariscope and the observation of 

 the position of coloured fringes of diffraction (in consequence 

 of interference) have been extensively employed in the prose- 

 cution of scientific inquiry. Meteorology has made equal 

 advances with physical astronomy in this new path. 



However diversified the power of vision may be in different 

 persons, there is nevertheless a certain average of organic 

 capacity, which was the same among former generations, as, 

 for instance, the Greeks and Romans, as at the present day. 

 The Pleiades prove that several thousand years ago, even as 

 now, stars which astronomers regard as of the 7th magnitude, 

 were invisible to the naked eye of average visual power. 

 The group of the Pleiades consists of one star of the 3rd 

 magnitude, Alcyone ; of two of the 4th, Electra and Atlas ; 

 of three of the 5th, Merope, Ma'ia, and Taygeta; of two 

 between the 6th and the 7th magnitudes, Pleione and Celreno ; 

 of one between the 7th and the 8th, Asterope ; and of many 

 very minute telescopic stars. I make use of the nomencla- 

 ture and order of succession at present adopted, as the same 

 names were amongst the ancients in part applied to other 

 stars. The six first-named stars of the 3rd, 4th and 5th magni- 

 tudes were the only ones which could be readily distinguished. 14 



14 Hipparchus says (ad Arati Phcen. 1, pag. 190, in Urano- 

 logio Petavii], in refutation of the assertion of Aratus, that 

 there were only six stars visible in the Pleiades : " One star 

 escaped the attention of Aratus. For when the eye is atten- 

 tively fixed on this constellation on a serene and moonless 

 night, seven stars are visible, and it therefore seems strange 



