REFLEXION AM) REFRACTION OF LIGHT. 37 



tage of connecting the velocity of propagation in dense bodies 

 directly with their constitution, and so of advancing a step in the 

 process of physical induction. On the other hand, it requires us 

 to admit that the particles of ether and those of gross bodies 

 exert no mutual action of any kind. We know too little of the 

 ether, or of its properties, to deny this, simply because it is unsup- 

 ported by any of the properties of matter hitherto revealed ; but it 

 must at the same time be admitted that the violation of such 

 analogies furnishes an argument of some weight against the theory 

 which demands them. 



Whatever supposition we may frame respecting the constitution 

 -of bodies, or of the ether within them, in the wave-theory, it must be 

 such that the velocity of propagation is less in the denser medium. 

 In the theory of emission, on the other hand, it is the reverse ; so 

 that although it conducts to the same result, it does so by an 

 opposite route. Here, then, the rival theories are at issue upon a 

 matter of fact ; and we have only to ascertain this fact, in order 

 to be able to decide between them. This seemed to be accom- 

 plished by the reasonings of Young. From the laws of inter- 

 ference it appears that homogeneous light, in its progress in space, 

 passes through certain periodically returning states, the intervals 

 -of which are constant in the same medium; while in different 

 media they are proportional to the velocities of propagation, 

 since the number of such intervals in a given quantity of light can- 

 not be supposed to vary. Now, it followed from the experiments 

 of Newton, that the intervals, by which he explained the pheno- 

 mena of thin plates, were diminished in the denser medium ; and 

 as these intervals have been shown by Young to be identical with 

 those deduced from the law of interference, it followed that the 

 velocity of light was slower in the denser medium.* Newton had 

 even found the ratio of the magnitudes of the intervals to be the 

 same with that of the sines of incidence and refraction ; and this 

 is precisely as it should be on the principles of the wave-theory. 



But the retardation of light in the denser medium has been 



quantity ~ is nearly constant, whatever be the compression. This result i a 



very simple consequence of the theory of emission ; its experimental truth has been 

 -established by MM. Biot and Arngo. Phil. Mag., New Series, vol. vii. 



* "Experiments and Calculations relative to Physical Optics." Phd. Tram., 

 1803. 



