on the Hypothesis of Undulations. 331 



covery, that this effect is produced even by rays more refrangible 

 than those of the visible spectrum. This property, without dimi- 

 nishing the intensity of the colours which are freely transmitted, 

 causes a peculiar dispersion to take place at the surface of the 

 medium, and from points contiguous to the surface, the quality 

 and quantity of which may be modified by the thickness of the 

 absorbing substance. Do these facts, it may be asked, prove 

 that a ray of definite refrangibility is composed of rays of differ- 

 ent colours ? A good theory of light ought to give an answer to 

 this question. Not being able to form the slightest conception 

 how a theory of light which makes the vibrations of individual 

 atoms of the sether the exponents of visible properties of light 

 can be brought to bear on this point, I venture to state that the 

 question may be answered if the undulatory theory be based on 

 hydro dynamical principles. In the pages of this Journal I 

 proved long since, by general hydrodynamical equations, that 

 the function which, as stated above, expresses the properties of 

 an uncompouuded ray, is not arbitrary, nor due to the mode of 

 disturbance, but arises out of relations of a compressible fluid to 

 time and space, which are shown by the analysis to be indepen- 

 dent of particular disturbances. Consequently any disturbance 

 proper for producing light will in general give rise to rays of all 

 refrangibilities. When a ray of the spectrum enters an absorb- 

 ing medium whose atomic arrangement does not allow of its 

 transmission, the ray-undulations must be broken up, and the 

 sether suffers disturbance by the action of the atoms at rest on 

 the undulations, as really as when atoms by their motion disturb 

 the sether at rest. According to this view, the generation of new 

 light in an absorbing medium does not prove that the intromitted 

 ray was compounded, since there is no resolution of the original 

 ray, but a new disturbance. Now I think that I am right in 

 saying that Sir David Brewster, in his examination of spectra 

 produced by absorbing media, received this new (epipolic) light 

 in conjunction with light freely transmitted ; and that, owing to 

 this circumstance, the colour of the latter appeared modified. 

 At least this inference seems to be justified by the experiments 

 of M. Helmholtz, who proved that the modifying light is decom- 

 posable by the prism, and got rid of the effect of it by insulating 

 as much as possible the freely transmitted light (see Phil. Mag. 

 S. 4. vol. iv. Dec. 1852). But M. Helmholtz does not appear 

 to be aware that the experiments he made to account for what 

 Sir David Brewster saw, were proper for getting rid of dispersed 

 light peculiar to absorbing media, which originates in the ab' 

 sorted rays and varies according to the degree of absorption, and 

 that the insulation of the rays transmitted by the media was 

 Successful in removing the dispersed light, just because these 



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