482 LECTURE XX. 



The investigations of ^Ir. 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 



liglit similar to that of Ilooke and Hiiygens, with an improvement originally 



suggested by Newton, respecting the nature of colours, I have endeavoured 



to obtain a satisfactory explanation of many circumstances, 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 excited, I trust that the science of optics will be 



essentially benefited, even if the theory should be ultimately confuted. 



