sect, v.] DISSERTATION SECOND. 195 



Oplicks, as indeed all the branches of natural philoso- 

 phy, have great obligations to Hnygens. The former was 

 among (he first scientifick objects which occupied his 

 mind ; and his Dioptricks, though a posthumous work, is 

 most of it the composition of his early youth. It is writ- 

 ten with great perspicuity and precision, and is said to 

 have been a favourite book with Newton himself. Though 

 beginning from the first elements, it contains a full deve- 

 lopment of the matters of greatest difficulty in the con- 

 struction of telescopes, particularly in what concerns the 

 indistinctness arising from the imperfect foci into which 

 rays are united by spherical lenses ; and rules are deduc- 

 ed for constructing telescopes, which, though of different 

 sizes, shall have the same degree of distinctness, illumina- 

 tion, &c. Htiygens was besides a practical optician; he 

 polished lenses, and constructed telescopes with his own 

 hands, and some of his object-glasses were of the enormous 

 focal distance of 130 feet. To his Dioptricks is added a 

 valuable treatise De Formandis Vitris. 



In the history of opticks, particular attention is due to 

 his theory of light, which was first communicated to the 

 Academy of Sciences of Paris, in 16fS, and afterwards 

 published, with enlargements, in 1690. ' 



Light, according to this ingenious system, consists in 

 certain undulations communicated by luminous bodies to 

 the etherial fluid which fills all space. This fluid is com- 

 posed of the most subtle matter, is highly elastick, and the 

 undulations are propagated through it with great velocity 

 in spherical superficies proceeding from a centre. Light, 

 in this view of it, differs from that of the Cartesian sys- 

 tem, which is supposed to be without elasticity, and to 

 convey impressions instantaneously, as a staff does from 

 the object it touches to the hand which holds if. 



' Traite de la Lumiere. Leyd. 1690, 



