716 COSMOS. 



understanding of its original and causal conditions.* Modern 

 times owe the discovery of these conditions, and the clear in- 

 sight into the laws, according to which (unpolarised) rays of 

 light emanating from one and the same source, but with a 

 different length of path, destroy one another and produce 

 darkness, to the successful penetration of Thomas Young. 

 The laws of the interference of polarised light were discovered 

 in 1816, by Arago and Fresnel. The theory of undulations 

 advanced by Huygens and Hooke, and defended by Leonhard 

 Euler, was at length established on a firm and secure- 

 basis. 



Although the latter half of the seventeenth century acquired 

 distinction from the attainment of a successful insight into the 

 nature of double refraction, by which optical science was so 

 much enlarged; its greatest splendour was derived from New- 

 ton's experimental researches and Olaus Romer's discovery, in 

 1675, of the measurable velocity of light. Half a century 

 afterwards, in 1728, this discovery enabled Bradley to regard 

 the variation he had observed in the apparent place of the stars 

 as a conjoined consequence of the movement of the earth in 

 its orbit, and of the propagation of light. Newton's splendid 

 work on Optics did not appear in English till 1704, having 

 been deferred, from personal considerations, till two years 

 after Hooke's death; but it would seem a well attested 

 fact that even before the years 1666 and 1667f he was in 



Eossession of the principal points of his optical researches, 

 is theory of gravitation and differential calculus (method of 

 fluxions). 



* On Grimaldi's and Hooke's attempt to explain the polarisation of 

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

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



t Brewster, The Life of Sir Isaac Newton, p. 17. The date of the year 

 1665 has been adopted for that of the invention of the method of flux- 

 ions, which, according to the official explanations of the Committee of 

 the Royal Society of London, April 24, 1712, is "one and the same with 

 the differential method, excepting the name and mode of notation." 

 "With reference to the whole unhappy contest on the subject of priority 

 with Leibnitz, in which, strange to say, accusations against Newton's 

 orthodoxy were even advanced, see Brewster, pp. 189-2] 8. The fact that all 

 colours are contained in white light was already maintained by De la 

 Chambre, in his work entitled "La Lumiere" (Paris, 1657), and by Isaac 

 Vossius, (who was afterwards a Canon at Windsor,) in a remarkable me- 



