332 cosmos. 



pied with the double refraction of light in crystals of Iceland 

 spar, i. e., with the separation of the pencils of light into two 

 parts, he also discovered, in 1678, that kind of polarization of 

 light which bears his name. The discovery of this isolated 

 phenomenon, which was not published till 1690, and, conse- 

 quently, only five years before the death of Huygens, was fol- 

 lowed, after the lapse of more than a century, by the great 

 discoveries of Malus, Arago, Fresnel, Brewster, and Biot.* 

 Malus, in 1808, discovered polarization by reflection from pol- 

 ished surfaces, and Arago, in 1811, made the discovery of col- 

 ored polarization. A world of wonder, composed of manifold 

 modified waves of light, having new properties, was now re- 

 vealed. A ray of light, which reaches our eyes, after travers- 

 ing millions of miles, from the remotest regions of heaven, an- 

 nounces of itself, in Arago' s polariscope, whether it is reflected 

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

 gaseous body ; announcing even the degree of its intensity. t 

 By pursuing this course, which leads us back through Huygens 

 to the seventeenth century, we are instructed concerning the 

 constitution of the solar body and its envelopes ; the reflected 

 or the proper light of cometary tails and the zodiacal light ; 

 the optical properties of our atmosphere ; and the position of 

 the four neutral points of polarization^ which Arago, Babinet. 

 and Brewster discovered. Thus does man create new organs, 

 which, when skillfully employed, reveal to him new views of 

 the universe. 



Next to polarization I should name the interference of light, 

 the most striking of all optical phenomena, faint traces of which 

 were also observed in the seventeenth century by Grimaldi 

 in 1665, and by Hooke, although without a proper understand- 

 ing of its original and causal. conditions. Modern times owe 

 the discovery of these conditions, and the clear insight into the 

 laws, according to which, (unpolarized) rays of light, emana- 

 ting from one and the same source, but with a different length 

 of path, destroy one another and produce darkness, to the suc- 

 cessful penetration of Thomas Young. The laws of the in 



* On the important law discovered by Brewster, of the connection 

 between the angle of complete polarization and the index of refraction, 

 see Philosophical Transactions of the Royal Society for the Year 1815 

 p. 125-159. t See Cosmos, vol. i., p. 39 and 52. 



t Sir David Brewster, in Berghansand Johnson's Physical Atlas, 1847 

 Part vii., p. 5 (Polarization of the Atmosphere). 



On Grimaldi's and Hooke's attempt to explain the polarization of 

 soap-bubbles by the interference of the rays ol light, see Arago, in th 

 Annuaire for 1831, p. 164 (Brewster's Life of Newton, p. 53). 



