Intelligence and Miscellaneous Articles, 387 



generally accepted, this experiment having in fact been frequently- 

 quoted as one of the props of the adverse or undulatory theory. 



It seems, however, rather surprising that any great weight should 

 be attached to an apparent disproval, by a single test, of one merely 

 imaginary function of corpuscular light, especially as the test itself 

 is utterly fallacious ; for who shall say that retardation by attraction 

 is the only possible means by which emitted light could be refracted ? 

 and how can we know that the two stars selected by Arago had 

 either no proper motion of their own, or none of a sort to affect his 

 result ? 



Perhaps the only cases in which we can be sure of receiving star- 

 light of absolutely different velocities are those of such binary stars 

 the plane of whose orbit is not at right angles with the line from 

 thence to the earth. When that line lies in the plane of such an 

 orbit, and the two stars are situated at right angles to that line, it 

 is clear that the velocity of the rays reaching us from one star ex- 

 ceeds that of the rays from the other by double the speed of those 

 stars in their orbit. That light reaches us at various velocities from 

 all the various stars is of course as certain as that they have proper 

 motions, or that our solar system is moving through space ; but the 

 difficulties in the way of gaining any accurate comparison of those 

 velocities are very great. 



In reflecting upon the undulatory theory of light, I have been 

 quite unable to conceive how the luminiferous aether could " tremble 

 laterally," as the phrase is, causing vibrations transverse to the line 

 of propagation, without a direct relation existing between those 

 lateral vibrations and the forward impulses by which the phenomena 

 of light are translated. The aether being continuous and material 

 and elastic, being, in a word, capable of sustaining a vibration, a 

 vibratory impulse in it at right angles to the course of a ray of light 

 seems fairly comparable to a lateral displacement of a point in a 

 rope, or to the merely vertical vibration in a water-wave not of trans- 

 lation, or to the vibration of air by a sound, all of which vibrations 

 produce (we may almost say ctre) longitudinal undulations. 



But if the lateral vibrations of the aether produce the onward pro- 

 pagation of the ray, then the number of lateral impulses in a second 

 equals the number of forward impulses arriving in a second at a 

 relatively stationary point ; and as it is established that the number 

 of lateral impulses varies according to the colour of the light, so the 

 number of forward impulses striking a relatively stationary point 

 (say a retina)]must vary with the colour of the light ; and if the source 

 of light, instead of being at a constant distance, should rapidly ap- 

 proach the retina, the latter must receive a greater number of 

 impulses per second, and its impression of colour therefore must be 

 correspondingly modified. If the retina and the source of light 

 rapidly separate from each other, the number of impulses striking 

 the retina must on the other hand be diminished, producing the cor- 

 responding change in the perceived colour. 



Now if we imagine a star emitting white light to approach us in 

 an orbital movement at a sufficient rate of speed, its light should 



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