CONFIRMATION OP THE UXBULATORY THEORY. 



towards perfection as a theory ; and this task we have now nearly exe 

 eutecl as far as our abilities allow. 



AYe have been desirous of showing that the type of this progr - -, 

 in the histories of the two great sciences, Physical Astronomy and 

 Physical Optics, is the same. In both we have many Laivs of Plie.no- 

 nnna detected and accumulated by acute and inventive men ; we have 

 Preludial guesses which touch the true theory, but which remain for 

 a time imperfect, undeveloped, unconfirmed : finally we have the Epoch 

 when this true theory, clearly apprehended by great philosophical 

 geniuses, is recommended by its fully explaining what it was first meant 

 to explain, and confirmed by its explaining what it was not meant to 

 explain. AYc have then its Progress struggling for a little while with 

 adverse prepossessions and difficulties ; finally overcoming all these, 

 and moving onwards, while its triumphal procession is joined by al! 

 the younger and more vigorous men of science. 



It would, perhaps, be too fanciful to attempt to establish a parallel- 

 ism between the prominent persons who figure in these two histories. 

 It' we were to do this, we must consider Huyghcns and Hooke as stand- 

 ing in the place of Copernicus, since, like him, they announced tl.< 

 true theory, but left it to a future age to give it development and 

 mechanical confirmation ; Mains and Brewstcr, grouping them together. 

 correspond, to Tycho Brahe and Kepler, laborious in accumulating 

 observations, inventive and happy in discovering laws of phenomena : 

 and Young and Fresnel combined, make up the Newton of optical science. 



[2nd Ed.] [In the Report on Physical Optics, (Brit. Ass. ^?ey/ 

 1834,) by Prof. Lloyd, the progress of the mathematical theory afu-i 

 Frcsnel's labors is stated more distinctly than I have stated it, to tV- 

 following effect. Ampere, in 1828, proved Fresnel's mathematical re- 

 sults directly, which Fresnel had only proved indirectly, and derived 

 from his proof Fresnel's beautiful geometrical construction. Prof. Mac 

 Cullagh not long after gave a concise demonstration of the same theo- 

 rem, and of the other principal points of Fresnel's theory. He repre- 

 sents the elastic force by means of an ellipsoid whose axes are inversely 

 proportional to those of Fresnel's generating ellipsoid, and deduces 

 Fresnel's construction geometrically. In the third Supplement to his 

 Essay on the Theory of Systems of Rays (Trans. R. I. Acad. v< 

 xvii.), Sir AY. Hamilton has presented that portion of Fresnel's theory 

 which relates to the fundamental problem of the determination < 

 the velocity and polarization of a plane wave, in a very elegant and 

 analytical form. This he does by means of what he calls the 



