REFLEXION AND REFRACTION OF LIGHT. 41 



tions, or by the length of the wave ;* and the addition has been 

 received by all its supporters. But observation proves that the 

 refractive index, or the ratio of the velocities of propagation, 

 in the two media, is different for the light of different colours. 

 The advocates of the wave-theory, therefore, are forced to con- 

 clude that the velocity of propagation in refracting media varies 

 with the length of the wave. Here, then, we encounter a difficulty 

 in this theory, which has been regarded as the most formidable 

 obstacle to its reception. Theory indicates that the velocity of 

 wave-propagation is constant in the same medium, depending 

 solely on the elasticity of the medium as compared with its density. 

 That velocity, therefore, should be the same for light of all colours, 

 as it is found to be for sound of all notes. 



Various attempts have been made to solve this difficulty, f 

 Euler thought that the successive waves underwent an increase of 

 velocity arising from their mutual action ; and this increase he 

 supposed to vary with their length, the waves of greatest length 

 undergoing the least augmentation of velocity, and being there- 

 fore most refracted. J But the phenomena of coloured rings, as 

 Euler perceived, compel us, on the contrary, to suppose that the 

 lengths of the waves diminish as the refrangibility increases ; and 

 he seems himself to have abandoned his first conjecture. 



Dr. Young accounted for dispersion by the supposition that 

 the solid particles of the refracting substance vibrate, as well as 

 the particles of the ether within it ; and that the former vibra- 

 tions affect the latter, and affect them differently according to 

 their frequency. Mr. Challis has adopted and developed this 

 hypothesis. According to this author, it has been already ob- 

 served, the diminished velocity of transmission in the denser 

 medium may be explained by the obstacle which the solid par- 

 ticles of the medium offer to the free movement of the ethereal 

 particles. If the former be supposed to be immovable, the ratio 

 of the velocities of propagation, in free space and in the medium, 



* Phil. Trans., 1672. 



t It is scarcely necessary to advert here to the law proposed by M. Rudberg, to 

 connect the lengths of an undulation, or the velocities of propagation, in different 

 media ; for this law is purely hypothetical, and its apparent consistency with obser- 

 vation has arisen solely from the adaptation of the arbitrary constants which enter 

 the expression. Annales de Chimie, torn, xxxvi., xxxvii. 



J Opuscula varii Aryiimcnti, torn. i. p. 217. 



