ON THE HISTORY OF OPTICS. 479 



consequence of a discussion with Euler, Klingenstierna, and some other 

 mathematicians, that Mr. DoUond was led to make experiments on the re- 

 fraction of different kinds of glass; these gentlemen had not questioned the 

 general truth of Newton's opinion respecting the dispersion of the different 

 colours, but Euler had asserted that the eye itself produced a refraction free 

 from the appearance of colour, and Klingenstierna had shown the possibility 

 of producing a deviation by refraction, without a separation of colour, ac- 

 cording to the laws of refraction laid down by Newton himself. When 

 Dollond had once discovered the material difference which exists between 

 the dispersive properties of flint glass and of crown glass, it was easy to 

 produce the combination recjuired; but this ingenious artist was not satis- 

 fied with the advantage of freedom from colours only; he adjusted the 

 forms and apertures of his lenses in the most skilful manner to the correction 

 of aberrations of various kinds, and he was also particularly fortunate in being 

 able to obtain, about the time of his discovery, a glass of a quality superior 

 to any that has been since manufactured. 



This opinion of Euler respecting the eye was, however, by no means well 

 founded, for the eye acts very differently on rays of different colours, as we may 

 easily observe by viewing a minute object in different parts of a beam of light, 

 transmitted through a prism. It must be allowed that this great mathema- 

 tician was less fortunate in his optical theories than in many other depart- 

 ments of science; his mathematical investigations of the effects of lenses are 

 much more intricate and prolix than the subject actually requires, and with 

 respect to the nature and propagation of light, he adopted several paradoxical 

 opinions. Assuming the theory of Huygens, with the additional hypothesis 

 respecting the nature of colours, which had been suggested by Newton, and 

 maintained by Pardies and Malebranche, that is, that the difference of co- 

 lours, like that of tones in music, depends on the different frequency of the 

 vibrations constituting light; he imagined that opaque bodies are not seen 

 by reflected light, but that their particles are agitated by the impulse of the 

 light which falls on them, and that the vibrations of these particles render 

 the bodies again visible in every direction; he also conceived that the undu- 

 lations of light are simply propagated through the solid substances of trans- 

 parent mediums, in the same manner as sound travels through the air. But 

 on these suppositions, all bodies would have the properties of solar phos- 



