THE UNIVERSE. DISCOVERIES IN THE CELESTIAL SPACES. 329 



this insulated phenomenon (which was not published until 

 1690, within five years of Huy gens' death) was followed 

 by the great discoveries of Malus, Arago, Eresnel, 

 Brewster ( 504 ) and Biot, Malus, in 1808, discovered po- 

 larisation by reflection from polished surfaces ; and Arago, 

 in 1811, discovered coloured polarisation. A world of 

 wonders of variously modified waves of light gifted with 

 new properties, was now opened. A ray of light which 

 reaches our eyes from the regions of space, from a heavenly 

 body many millions of miles distant, when received in Arago's 

 polariscope, tells as it were of itself whether it is reflected or 

 refracted, whether it emanates from a solid, a fluid, or a 

 gaseous body, ( 505 ) and even announces its degree of inten- 

 sity. Advancing in this path, which takes us back through 

 Huygens to the 17th century, we are instructed respecting 

 the constitution of the solar orb and its envelopes, the 

 reflected or the proper light of the tails of comets and of the 

 Zodiacal Light, the optical properties of our atmosphere, 

 and the position of the four neutral points of polarisation, ( 5o6 ) 

 which Arago, Babinet, and Brewster discovered. Thus man 

 makes for himself, as it were, new organs, which, when 

 skilfully used, open to him new views of nature. 



We should next name, by the side of the polarisation of 

 light, the most striking of all the phsenomena of optics the 

 phsenomenon of ' c interferences," faint indications of which 

 were also observed in the 17th century, though without any 

 understanding of their causal conditions, by Grimaldi, in 

 1665, and by Hooke. ( 507 ) Our own time is indebted for the 

 discovery of these conditions, and the clear recognition of 

 the laws according to which rays of light (unpolarised)r. 

 when they proceed from one and the same source, but with a 



