370 



E. B. Hunt on the Dispersion of Light. 



distinguished naturalist has accumulated many equally striking 

 proofs of the minuteness of this and other types of organic exist- 

 ences. More than 1500 of those " partitions of monad-bellies," 

 would be required to make up by their thickness, the mean length 

 of a luminous wave, or l-5(J,000th of an inch. And can we 

 doubt that these partitions are actually organized and that those 

 eyes actually see ? And does any one suppose the particles of 

 ether, by which such vision must be effected, larger or more wide- 

 ly separated than atoms? But the length of a wave in a solid 

 must include many thousands of atoms. The more we ponder 

 on these fads, the more completely inadmissible does the physi- 

 cal hypothesis of finite intervals become. Not only does it lead 

 to a result conflicting with fact, but it is, in itself most difficult 

 to concede. Are we not then, in view of all the facts, justly 

 entitled to infer, that dispersion remains unexplained in the un- 

 dulatory system. 



It may seem an inexcusable rashness, to express dissatisfaction 

 with a demonstration so long honored, as that given by Huy- 

 ghens for the law of refraction. Yet thinking that, though clear 

 and sound and beautiful in mathematics, it is radically unsound 

 in mechanics and wholly false to nature, I would ask a candid 

 hearing to an honest argument. This demonstration turns on 

 the retardation of velocity of plane waves entering a denser medi- 

 um, with its converse, and on the composition of the primary 

 wave by the series o( secondary waves. 



The wave theory, no less than that of emission, makes light 

 to consist in a series of mechanical impulses, whose general tra- 

 jectory in the same medium, is rectilinear. Now a simple and 

 direct retardation or acceleration of such an impulse could not 

 change the direction of its trajectory : another force or impulse 

 must combine with this in order that its course may be broken 

 or curved. I see not how refraction or flexure of luminous tra- 

 jectives can result from mere retardation. Change of velocity 

 cannot of itself produce change of direction. Refraction takes 

 place at the dividing surface of media. If any oblique action be 

 ascribed to such a surface as an elastic surface of impact, it should 

 be to produce a flexure in the contrary direction to the observed 

 refraction. Assuming a plane wave, perpendicular to a beam, as 

 made up of an infinite number of elementary impulses, and 

 obliquely incident on a denser or rarer medium ; each impulse 

 should according to all mechanical principles, continue on in * 

 straight line, falling behind or advancing beyond the uniformly 

 moving perpendicular plane, according to the time and rate oj 

 retardation or acceleration. The actual surface of impulses would 

 thus become oblique to the direction of the beam, this direction 

 remaining constantly unchanged. The postulate that the direc- 

 tion of the ray and the normal to the wave are identical, is whol- 





