830 EPOCHS IN THE HISTORY OF THE CONTEMPLATION OP 



different length of path, destroy each other and produce 

 darkness, to the acute and successful penetration of Thomas 

 Young. The laws of the interference of polarised light 

 were discovered in 1816, by Arago and Eresnel. The 

 theory of undulations, advanced by Huygens and Hookc. 

 and defended by Euler, at last found a firm basis. 



But if the latter half of the 17th century was distin- 

 guished by an important enlargement, of optical knowledge, 

 in the attainment of an insight into the nature of double re- 

 fraction, it has been invested with a far higher splendour by 

 Newton's experimental researches, and by Olaus Homer's dis- 

 covery (in 1675) of the measurable velocity of light; a dis- 

 covery which enabled Bradley, half a century later (in 1728), 

 lo regard the variation which he found in the apparent place 

 of the stars as a consequence of the movement of the earth 

 in her orbit combined with the propagation of light. New- 

 ton's Optics appeared in 1704, not being published in 

 English for personal reasons until two years after Hooke's 

 death ; but this magnificent work may be regarded as be- 

 longing to the 17th century, for we are assured that, even 

 previously to the years 1666 and 1667, its great author was 

 in possession. ( 508 ) of the essential points of his optical dis- 

 coveries, of his theory of gravitation, and of the method of 

 fluxions. 



In order not to break the links of the common bond 

 which unites the general " primitive phaenomena of matter/' 

 I place here, immediately after the above brief notice of 

 Huygens, Grimaldi, and Newton, considerations on terres- 

 trial magnetism and atmospheric temperature, so far at 

 least as the foundations of these studies were established in 

 the century which it is the object of this section to describe. 



