ON THE HISTORY OF OPTICS. ' 475" 



seen Snellius's papers, although he published the law of refraction without 

 acknowledging to whom he was indebted for it. Descartes also explained 

 the formation of the secondary rainbow, and truly determined the angular 

 magnitude of both the bows from mathematical principles; he did not, how- 

 ever, give a sufficient reason for the production of colours in either case. 

 Descartes imagined light to consist in motion, or rather pressure, transmit- 

 ted instantaneously through a medium infinitely elastic, and colours he at- 

 tribsted to a rotatory motion of the particles of this medium. He supposed 

 that light passed more rapidly through a denser medium than through a rarer; 

 other philosophers about the same time maintained a contrary opinion, with- 

 out deciding with respect to any general theory of light : thus Fermat and 

 Leibnitz deduced, on tliis supposition, the path of refracted light from the 

 natural tendency of every body to attain its end by the shortest possible way; 

 and Barrow derived the same law, in a more geometrical manner, from a simi- 

 lar hypothesis respecting the velocity of light, by considering a pencil of 

 light as a collection of collateral rays influencing each other's motions. 

 We are indebted to this learned mathematician for the first accurate inves- 

 tigation of the properties of refracting and reflecting surfaces, and for the 

 most general determination of the situations of focal points. 



The industrious Mr. Boyle had noticed with attention the phosphorescence of 

 diamonds, the colours produced by the eflfect of scratches on the surfaces of 

 polished metals, and the diversified tints which a bubble or a film of soapy 

 water usually assumes. His'assistant, Dr. Hooke, investigated these and 

 other similar appearances with still greater accuracy, and proposed, in his 

 Micrographia, which was published in 1665, a theory of light considerably 

 resembling that of Descartes: he supposes that light is an impulse propagated 

 through a medium highly, but not infinitely, elastic; that refraction is pro- 

 duced by the readier transmission of light through the denser medium, and 

 that difference of colour consists in the different law of the particular im- 

 pulse constituting coloured light, so that red and blue differ from each other 

 in the same manner as the sound of a violin and of a flute. He explained 

 the colours of thin plates from the interference of two such pulses partially 

 reflected from the upper and under surface; but the hypothesis which he 

 assumed, respecting the nature of colours, renders this explanation wholly 



