Oisr THE HISTORY OF OPTICS. , 477 



substances, in which the phenomena are reduced to their simplest forms, 

 and of a collection of miscellaneous experiments on the colours produced in 

 cases of inflection or diffraction. 



With respect to the nature of light, the theory which Newton adopted 

 was materially different from the opinions of most of his predecessors. He 

 considered indeed the operation of an ethereal medium as absolutely neces- 

 sary to the production of the most remarkable effects of light, but he denied 

 that the motions of such a medium actually constituted light; he asserted, on 

 the contrary, that the essence of light consisted in the prv)jection of minute 

 particles of matter from the luminous body, and maintained that this pro- 

 jection was only accompanied by the vibration of a medium as an accidental 

 circumstance, which was also renewed at the surface of every refractive or 

 reflective substance. 



In the mean time Bartholin had called the attention" of naturalists and 

 opticians to the singular properties of the Iceland crystal, and had hastily 

 examined the laws of its unusual refraction. On this subject Huygens had 

 been much more successful-: his analysis of the phenomena of the double re- 

 fraction is a happy combination of accurate experiment with elegant theory; 

 it was published in I690, making apart of his treatise on light, the funda- 

 mental doctrines of which he had communicated to the Academy of Paris in 

 1678. They scarcely differ in their essential parts from those of our country- 

 man Dr. Hookc, but the subject of colours Huygens has left wholly un- 

 touched. Roemer had then lately made the discovery of the immense velo- 

 city with which light passes through the celestial regions, by observing the 

 apparent irregularities of the eclipses of Jupiter's satellites; and Huygens 

 readily admitted this property into his system; althougli Hooke, by a 

 singular caprice, professed himself more ready to believe that the propagation 

 of light might be absolutely instantaneous, than that its motion could be 

 successive, and yet so inconceivably rapid. The merits of Huygens in the 

 mathematical theory of optics were no less considerable than in the investi- 

 gation of the nature of light; his determinations of the aberrations of lenses 

 were the first refinement on the construction of telescopes. 



In the year 1720 Dr. Bradley had the good fortune to discover both the 



