PROPAGATION OF LIGHT PRINCIPLE OP INTERFERENCE. 29 



the phenomenon, but even in its minutest 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 measurements have shown that it is so. 

 The constant differences just alluded to are far too minute to be 

 directly measured ; but they can be calculated with great accu- 

 racy, when the distances of the successive bands from the central 

 one have been obtained. The latter distances have been deter- 

 mined by Fresnel with much nicety by micrometrical measure- 

 ments; 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 quantities 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 posi- 

 tion is therefore independent of the length of a wave, the rays of 

 all colours will be united there, and the fringe itself will be ichite, 

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

 and by most observers who have repeated the experiment. Mr. 

 Potter states, however, as the result of his observations, 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 circumstances which have oc- 

 casioned the variation in the results have been fully investigated 

 and understood. 



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

 periment of Fresnel, been admitted on all hands ; and the phe- 

 nomena 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, referred 

 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 regarded 

 as a weighty argument in favour of that theory. It now remains 



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



