SOI FOURTH REPORT — 1834. 



common origin, and are separated simply by reflexion at plane 

 surfaces, without any attending circumstance which can, by 

 possibility, be supposed to influence the result. The pheno- 

 menon is thus divested of everything nonessential, and it 

 becomes impossible to hesitate about its nature. But the ac- 

 cordance of theory and experiuient is maintained, not only in 

 the general features of the phenomenon, but even in its minu- 

 test details. The distances of the points of each fringe from the 

 two foci of reflected rays should, according to theory, differ by 

 a constant quantity, — that constant being an odd multiple of the 

 length of half a wave for the dark fringes, and an even multiple 

 of the same quantity for the bright ones. Hence the fringes 

 should be propagated in hyperbolic lines, whose foci are the 

 foci of the reflected pencils ; — and the most accurate measure- 

 ments have shown that it is so. The constant diffei-ences jus; 

 alluded to are far too minute to be directly measured ; but they 

 can be calculated with great accuracy, when the distances of the 

 successive bands from the central one have been obtained. The 

 latter distances have been determined by Fresnel with much 

 nicety by micrometrical measurements ; and the lengths of the 

 waves of each species of simple light, thence computed, agree 

 in the most satisfactory manner with the values of the same quan- 

 tities as deduced from the observation of Newton's rings. 



The central fringe is formed at those points in arriving at 

 which the two pencils have traversed equal paths ; and as its 

 position is therefore independent of the length of a wave, the 

 rays of all colours will be united there, and the fringe itself will 

 be white, or colourless. Such is the fact, as described by Fresnel 

 himself, and by most observers who have repeated the experi- 

 ment. Mr. Potter states, however, as the result of his obser- 

 vations, that the central fringe may be seen both black and white, 

 although more frequently the former ; and he urges the fact in 

 opposition to the wave- theory*. But it seems premature to 

 draw any inference from such experiments, until the circum- 

 stances which have occasioned the variation in the results have 

 been fully investigated and understood. 



The interference of the rays of light has, since the decisive 

 experiment of Fresnel, been admitted on all hands ; and the 

 phenomena which were previously explained on the Newtonian 

 hypothesis of the " fits of easy reflexion and transmission," are 

 now, by most of the advocates of the Newtonian theory, referi'ed 

 to this simpler and more fertile principle. This principle is, 

 it has been stated, an immediate and necessary consequence of 

 the wave-theory, and its experimental establishment must be 

 * Phil. Mag.. (3rd Series,) vol. ii. p. 280. 



