DISCOVEBIES IN THE CELESTIAL SPACES. 715 



exercised direct and special influence on general, or in other 

 words, on cosmical views of nature. With reference to the pro- 

 cesses of light, heat, and magnetism, I would first name Huy- 

 gens, Galileo, and Gilbert. While Huygens was occupied with 

 the double refraction of light in crystals of Iceland spar, i. 0., 

 with the separation of the pencils of light into two parts, he also 

 discovered in 1 678 that kind of polarisation of light which bears 

 his name. The discovery of this isolated phenomenon, which 

 was not published till 1690, and consequently only five years 

 before the death of Huygens, was followed, after the lapse of 

 more than a century, by the great discoveries of Malus, Arago, 

 Fresnel, Brewster, and Biot.* Malus, in 1808, discovered 

 polarisation by reflection from polished surfaces, and Arago in 

 1811, made the discovery of coloured polarisation. A world 

 of wonder composed of manifold modified waves of light, 

 having new properties, was now revealed. A ray of light 

 which reaches our eyes, after traversing millions of miles, 

 from the remotest regions of heaven, announces of itself in 

 Arago' s polariscope, whether it is reflected or refracted, whe- 

 ther it emanates from a solid, or fluid, or gaseous body ; an- 

 nouncing even the degree of its intensity. f By pursuing this 

 course, which leads us back through Huygens to the seven- 

 teenth century, we are instructed concerning the constitution 

 of the solar body and its envelopes; the reflected or the pro- 

 per light of cometary tails and the zodiacal light ; the optical 

 properties of our atmosphere; and the position of the four 

 neutral points of polarisation J which Arago, Babinet, and 

 Brewster discovered. Thus does man create new organs which 

 when skilfully employed, reveal to him new views of the 

 universe. 



Next to polarisation, 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 Gri- 

 maldi in 1665, and by Hooke, although without a proper 



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

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

 see Philosophical Transactions of the Royal Society for the year 1815, 

 pp. 125-159. 



t See Cosmos, pp. 18 and 33. 



J Sir David Brewster, in Berghans and Johnson's Physical Atlas, 

 1847, part vii. p. 5 (Polarisation of the Atmosphere). 



