kine:tic or mechanical view of nature. 13 



dilation, to be accumulated. A real physical theory, 

 however, was impossible until the notions suggested 

 by common - sense were completely reversed, and an 

 ideal construction put in the place of a seemingly 

 obvious theory. This was done in astronomy at one 

 stroke by Copernicus ; in optics only gradually, tenta- 

 tively, and hesitatingly. The purely geometrical rela- 

 tions of straight lines, which light seemed to resemble ; 

 of pencils of rays, which were bent back or altered in 

 their direction at the surface of plane or curved mirrors 

 and of transparent bodies ; seemed to flow quite easily 

 and naturally when in the seventeenth century the 

 simple law of refraction had been added to that of 

 reflexion, known already to the ancients. The sciences 

 of catoptrics and dioptrics, with their application to the 

 telescope and microscope, were thus so complete and 

 useful that to many it must have seemed ditticult and 

 unnecessary to plunge into a new theory ; ^ especially 



' It has always been the aim of process" (Tait, 'Light,' "ind ed.. p. 



" geometrical optics " to free itself 160). Owing to the difBculties 

 from every hyi)othesis on the physi- i which liave more and more jire- 

 cal nature of light, and to deduce ' sented themselves in the fundamen- 



properties of light from a tew simple tal conceptions f)f the wave-theory 



geometrical const ructions. Precisely and the vibrating ether, of whicli 



in the same way all geometrical and we shall learn more in the sequel of 



many jjhysical i)roperties of the this chapter, the desire to bring the 



stellar system can be deduced from j)henomena of refraction under a 



the kinematical formula of attrac- purely geometrical formula, and to 



tion, without discussing the nature emancipate the optics of crj'stals 



of gravitation. This desideratum from physical hypotheses, has be- 



— so far as optics is concerned — come very pronounced. Huygens' 



was before the mind of .Sir W. U. geometiical coustiut-tioM of the 



Hamilton, when, during tlie yeai's ordinary and extraordinary raj'.s in 



1824-33, he discovered and elabor- uniaxial crystals answered well, 



ated the theory of the " character- For luaxial crystals Fresnel had in- 



istic function, by the helj) of which troduced the wave-surface, towliicli 



all optical problems, whether on the corresponds Hamilton's i-haracter- 

 corjmscular or on the uiidulatory ' istic function. For didactic pur- 



tlieory, are solved by one coniiium poses, and for the practical applica- 



