PORTION OF THE COSMOS. VISUAL POWER. 47 



covery of polarisation. ( 97 ) Since that event, the more deeply- 

 examined phsenomena of double refraction, ordinary (Huyge- 

 nian) polarisation and coloured polarisation, interference, and 

 diffraction, have furnished the investigator with unexpected 

 means of distinguishing between direct and reflected light ( 98 ), 

 of penetrating the secret of the constitution of the Solar 

 orb and his luminous envelopes ("), of measuring the pres- 

 sure and the minutest aqueous contents of the atmosphere, 

 of discerning the bottom of the sea and its shoals by the 

 aid of a plate of tourmaline ( 10 ), and even of comparing, 

 according to Newton's example, the chemical ( 101 ) composi- 

 tion of several substances ( 102 ) with their optical effects. It 

 is sufficient to mention the names of Airy, Arago, Biot, 

 Brewster, Cauchy, Faraday, Fresnel, John Herschel, Lloyd, 



Malus, Neumann, Plateau, Seebeck to recall to 



the recollection of the scientific reader a series of brilliant 

 discoveries, and of the happiest applications of each newly- 

 discovered step. The great works of Thomas Young, 

 marked with the stamp of genius, more than prepared the 

 way for these important labours. Arago's polariscope, and 

 the observed position of coloured diffraction -fringes (conse- 

 quences of interference), have become of great and varied 

 use in the investigation. Meteorology has profited no less 

 than physical astronomy by the opening of this new path 

 of research. 



Different as is the power of vision with the naked eye in 

 different men, yet here also there is a certain mean degree of 

 organic capability, which was the same among the ancient 

 Greeks and Romans as in the present day. The Pleiades 

 furnish the proof of this, showing that some thousand years 

 ago, as now, stars which astronomers call of the 7th magnitude 



