380 LECTURE XL. 



stierna, and some other mathematicians, that Mr. Dollond was led to make 

 experiments on the refraction 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 Klingen- 

 stierna* had shown the possibility of producing a deviation by refraction, 

 without a separation of colour, according to the laws of refraction laid 

 down by Newton himself. When Dollond had once discovered the mate- 

 rial difference which exists between the dispersive properties of flint glass 

 and of crown glass, it was easy to produce the combination required ; but 

 this ingenious artist was not satisfied with the advantage of freedom from 

 colours only ; he adjusted the forms and apertures of his lensts 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 mathematician was less fortunate in his optical theories than in 

 many other departments 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 colours, 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 direc- 

 tion ; he also conceived that the undulations of light are simply propagated 

 through the solid substances of transparent mediums, in the same manner 

 as sound travels through the air. But on these suppositions, all bodies 

 would have the properties of solar phosphori, and the refraction of the 

 rarest of natural bodies would be incomparably greater than that of the 

 densest is actually found to be : and on the whole, although the character 

 of Euler has been so highly and so deservedly respected as to attach a cer- 

 tain degree of authority to all his opinions, so that in this instance the 

 name of Huygens has been almost superseded by that of Euler, yet in fact 

 he has added no argumentative evidence whatever to the theory, but, by 

 inaccurate and injudicious reasoning, has done a real injury to the cause 

 which he endeavoured to support. 



The researches of Lambert t may be considered as a continuation of 



* Schwed. Abhand. 1754, xvi. 300; 1760, xxii. 79. Ph. Tr. 1760, p. 944 ; and 

 Tentameu de Corrig. Aberrat. Luminis, 4to, Petrop. 1762. 

 f Photometria, Augsb. 1760. 



