366 E. B. Hunt on the Dispersion of Light. 



was wholly independent of the nature of the wave. But as re- 

 fraction is ascribed to a change of velocity in passing through 

 different media, and as each color in this change of media, enter- 

 ing along the same or parallel lines should pass through precisely 

 the same succession of media, there seemed no possible reason 

 why one ray should be more retarded or accelerated than another. 

 1 Admitting Huyghens' explanation of refraction, there seemed to 

 be no possibility of dispersion. Besides the analogy of sound, 

 from which the wave theory is so largely drawn, is entirely 

 averse to dispersion as due to unequal velocities : for all sounds 

 are heard in the same original order at all distances, and have 

 therefore equal velocities. Sir John Herschel, in alluding to this 

 " great difficulty," says : " it must not be dissembled that it is im- 

 possible to look on it in any other light than as a most formida- 

 ble objection to the undulatory doctrine." Yet the explanatory 

 powers of this doctrine are such as to have exalted it into the 

 list of physical generalizations. 



Fresnel and Cauchy have brought forward and prosecuted the 

 hypothesis of finite intervals to meet this difficulty. By supposing 

 the interval between the ether particles, a sensible distance as com- 

 pared with the lengths of luminous waves, a relation has been de- 

 duced between the length of wave and its velocity of transmission. 

 The spectral rays having unequal wave lengths, should then have 



unequal velocities, retardations and deviations. This explanation 

 turns wholly on a difference in the actual velocities of the differ- 

 ent rays, and has no application if this difference does not exist. 

 Assuming the correctness of the reasoning of these'eminent math- 

 ematicians, its application to nature rests wholly on the question, 

 whether the hypothesis of finite intervals be a fact ; also, if the 

 colored rays be proved to have actual equal velocities in the same 

 medium, this hypothesis cannot represent nature. In this case 

 too, nearly all the proposed explanations of dispersion become in- 

 admissible. That suggested by Dr. Young and developed by 

 Mr. Challis, of a resistance due to molecular inertia, would be ex- 

 cepted. This however seems not to have met with favor, and 

 like its companions contains the defect of confounding change of 

 velocity with change of direction. 



In view of the general history of physical optics, the following 

 propositions seem fully warranted. 



1. The undulatory theory of light, in its application to the ex- 

 planation of a vast range of observed optical facts, is eminently 

 successful. 



2. The Newtonian or emission theory, though perfectly repre- 

 senting many of the facts of light, entirely fails in many cases. 



3. The unequal refrangibility or spectral dispersion of light, is 

 the weak point of the undulatory, and the strong point of the 

 emission theory. 



