REPORT ON PHYSICAL OPTICS. 315 



coloured spectrum, whose length is parallel to the direction of 

 the earth's motion. 



According to the modern advocates of the theory of emission, 

 the molecules of light are heterogeneous ; and the attractions 

 exerted on them by bodies vary with their nature, and are, in 

 this respect, analogous to chemical affinities. This supposition, 

 however, as Dr. Young has justly observed, is but veiling our 

 inability to assign a mechanical cause for the phenomenon. 



It is remarkable that Newton himself was the first to suggest 

 that part of the wave-theory, in which the colour of the light is 

 supposed to be determined by the frequency of the ethereal vibra- 

 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 

 toith the length of the wave. Here, then, we encounter a diffi- 

 culty in this theory, which has been regarded as the most for- 

 midable 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 therefore most refracted |. But the phenomena of coloured 

 rings, as Euler perceived, compel us, on the contrary, to sup- 

 pose that the lengths of the waves dimmish 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 



« P/iil. 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 consist- 

 ency with observation has arisen solely from the adaptation of tlie arbitrary 

 constants which enter the expression. — ^finales de Chimie, tom. xxxvi. xxxvii. 



J Opuscula varii Jrgumenti, tom. i. p. 217. 



