346 Mr. G. G. Slokns 07i (he Constitution of 



pressions does not tend, as ju. vanishes, to become one for 

 which udx-\- ...is an exact differential, and therefore the motion 

 which would be obtained by supposing uiLv + ... an exact 

 differential, and applyinor to the a2ther the common equations 

 of hydrodynamics, would be unstable. The proof supposes 

 the motion in question to be steady ; but such it may be proved 

 to be, if the velocity of the earth be regarded as uniform, and 

 an equal and opposite velocity be conceived impressed both 

 on the earth and on the aether. Hence the stars would appear 

 to be displaced in a manner different from that expressed by 

 the well-known law of aberration. 



When, however, we take account of a tangential force in 

 the Eether, depending, not on relative velocities, or at least not 

 on relative velocities only, but on relative displacements, it 

 then becomes possible, as I have shown (Phil. Mag., vol. xxix. 

 p. 6), to explain not only the perfect regularity of the motion, 

 but also the circumstance that udx+ ... is an exact differential, 

 at least ior the aether which occupies free space ; for as regards 

 the motion of the Eether which penetrates the air, whether 

 about the limits of the atmosphere or elsewhere, I do not 

 think it prudent, in the present state of our knowledge, to 

 enter into speculation ; I prefer resting in the supposition that 

 udiV-\-,.. is an exact differential. According to this explana- 

 tion, any nascent irregularity of motion, any nascent deviation 

 from the motion for which udx + ... is an exact differential, is 

 carried off into space, with the velocity of light, by transversal 

 vibrations, which as such are identical in their physical nature 

 with light, but which do not necessarily produce the sensation 

 of light, either because they are too feeble, as they jn-obably 

 would be, or because their lengths of wave, if the vibrations 

 take place in regular series, fall beyond the limits of the visible 

 spectrum, or because they are discontinuous, and the sensa- 

 tion of light may require the succession of a number of similar 

 vibrations. It is certainly curious that the astronomical phag- 

 nomenon of the aberration of light should afford an argument 

 in support of the theory of transversal vibrations. 



Undoubtedly it does violence to the ideas that we should 

 have been likely to form u jyiiori of the nature of the asther, 

 to assert that it must be I'egarded as an elastic solid in treating 

 of the vibrations of light. When, however, we consider the 

 wonderfal simplicity of the explanations of the phenomena of 

 polarization when we adopt the theory of transversal vibra- 

 tions, and the difficulty, which to me at least appears quite 

 insurmountable, of explaining these phoenomena by any vibra- 

 tions due to the condensation and rarefaction of an elastic 

 fluid such as air, it seems reasonable to suspend our judge- 



