DISCOVERIES IN THE CELESTIAL SPACES. 333 



terference of polarized light were discovered in 1816 by Ara- 

 go and Fresnel. The theory of undulations advanced by Huy- 

 gens 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 acquir- 

 ed distinction from the attainment of a successful insight into 

 the nature of double refraction, by which optical science was 

 so much enlarged, its greatest splendor was derived from New- 

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

 in 1675, of the measurable velocity of light. Haifa century 

 afterward, 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 1667,* he was in 

 possession of the principal points of his optical researches, his 

 theory of gravitation and differential calculus (method of flux- 

 ions). 



In order not to sever the links which hold together the gen- 

 eral primitive phenomena of matter in one common bond, I 

 would here immediately, after my succinct notice of the op- 

 tical discoveries of Huygens, Grimaldi, and Newton, pass to 



* 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 

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

 of the Royal Society of Loudon, April 24, 1712, ia "one and the same 

 with the differential method, excepting the name and mode of nota 

 tion." With reference to the whole uuhappy contest on the subject of 

 priority with Leibnitz, in which, strange to say, accusations against 

 Newton's orthodoxy were even advanced, see Brewster, p. 189-218. 

 The fact that all colors are contained in white light was already main- 

 tained by De la Chambre, in his work entitled "La Lumiere" (Paris, 

 1657), and by Isaac Vossius (who was afterward a canon at Windsor), 

 in a remarkable memoir entitled " De Lncis Natura et Proprietate" 

 (Amstelod., 1662), fur the knowledge of which I was indebted, two 

 years ago, to M. Arago, at Paris. Brandis treats of this memoir in the 

 new edition of Gehler's Physikalische Worferbvck, bd. iv. (1827), 8. 43, 

 and Wilke notices it very fully in his Gesch. der Optik, th. i. (1838), 

 s. 223, 228, and 317. Isaac Vossius, however, considered the funda- 

 mental substance of all colors (cap. 25, p. 60) to be sulphur, which 

 forms, according to him, a component part of all bodies. In Vossii Re- 

 gponsum ad Objecta, Joh. de Bruyn, Professoris Traject.ini, et Petri Petiti, 

 1663, it is said, p. 69, Nee lumen ullum est absque calore, nee calor ul- 

 lus absque lumiue. Lux sonus, anima (!) odor, vis magnetica, quamvia 

 incorporea, suut tamen aliquid. (De Lucis Nat., cap. 13, p. 29.) 



