382 LECTURE XL. 



Dr. Robert Darwin's* investigation of the effects of strong lights on the eye 

 appears to comprehend almost all possible varieties of these ocular spectra, 

 but it does not lead to any fundamental analogy, capable of explaining the 

 most intricate of them. 



The phenomena of the unusual atmospheric refraction, which frequently 

 produces double or triple images of objects seen near a heated surface, have 

 been successively illustrated by Mr. Huddart,t Mr. Vinc,^ and Dr. Wol- 

 laston, so that at present there appears to be little doubt remaining with 

 respect to their origin. Dr. Wollaston's instrument for the measurement 

 of refractive densities, very much facilitates the examination of the optical 

 properties of substances of various kinds : he has applied it very success- 

 fully to the confirmation of Huygens's theory of double refraction^; he has 

 corrected the common opinion respecting the division of the prismatic 

 spectrum ; he discovered, without being acquainted with the observations 

 of Ritter, the dark rays which blacken the salts of silver ; and he has re- 

 marked a singular property in some natural as well as artificial crystals, 

 which appear of one colour when viewed in the direction of the axis, and 

 of another when in a transverse direction. 



To Dr. Herschel the sciences of optics and astronomy are equally in- 

 debted. He has carried the construction of the reflecting telescope to a 

 degree of perfection, far exceeding all that had been before attempted, and 

 the well known improvements which astronomy has derived from his ob- 

 servations are numerous and important. In the course of his researches 

 for the attainment of his more immediate objects, he has also had the good 

 fortune to discover the separation of the rays of heat from those of light by 

 means of refraction ; a fact which has been sufficiently established by the 

 experiments of several other persons. 



The investigations of Mr. Laplace, relating to atmospherical refraction, 

 may be considered as the latest application of refined mathematics to the 

 purposes of optics and of astronomy. I have myself attempted to attain a 

 degree of certainty, in attributing the changes of the refractive powers of 

 the eye to a variation in the form of the crystalline lens ; I have discovered 

 a general law of the mutual action of two portions of light interfering with 

 each other, to which no exception has yet been shown ; and by reviving a 

 theory of light similar to that of Hooke and Huygens, with an improve- 

 ment originally suggested by Newton, respecting the nature of colours, 1 

 have endeavoured to obtain a satisfactory explanation of many circum- 

 stances, which appear, upon a minute examination, to be in every other 

 hypothesis difficulties absolutely insuperable. It cannot be expected that 

 all objections to such a system will at once be silenced, but if a full and 

 candid discussion only of the facts which I have advanced, should be ex- 

 cited, I trust that the science of optics will be essentially benefited, even 

 if the theory should be ultimately confuted. 



* Ph. Tr. 1786, p. 313. > Ph. Tr. 1797, p. 29. 



t Ph. Tr. 1799, p. 13. Ibid. 1800, p. 239 ; 1803, p. 1. 



