186 Prof. Challis on a Theory of the Zodiacal Light. 



this theory for concluding that motions expounded by terms of 

 the third order would be adequate to produce, excepting in very 

 small degree, the former phenomenon. I think also that the 

 origins of both luminosities ought to be looked for in dis- 

 turbances of vibratory motions, rather than in those of steady 

 motions. If we supposed the luminosity to be due to a kind of 

 reflexion of the solar rays from the parts of the aether subject to 

 the gyratory motion, we should, it is true, have disturbances of 

 vibratory motions expressed by terms of the order of the square 

 of the velocity, and might thus account for the zodiacal light, but 

 the phenomenon which is the immediate subject of this inquiry 

 would still be left unexplained. 



I come now to a consideration which, if I had proceeded con- 

 sistently with the general theory of the physical forces which I 

 expounded some time since in this Magazine, would have been 

 introduced at first. It is the characteristic of a true physical 

 theory that it fails only when its consequences are not rigidly 

 and completely followed out, whereas a false or imperfect theory 

 breaks down under this very process. It may be remembered 

 that in that general theory the different physical forces are 

 accounted for by the dynamical action of different orders of un- 

 dulations of the ethereal medium, and that the force of gravity 

 was ascribed to undulations emanating from masses, and of 

 much larger dimensions than those which produce the pheno- 

 mena of light. Now such large undulations emanating from 

 the sun would be subject to disturbances expressed by terms of 

 the second order in respect to velocity in their passage through 

 those portions of the aether which are in gyratory motion ; and 

 disturbances of this kind might, as I have elsewhere argued, be 

 made up of vibratory motions of different orders, a portion of 

 which may be presumed to be of the light-producing order. 

 But if we take into account the gravity-undulations emanating 

 from the sun, it is evident that we must also take account of 

 those emanating from the earth. Also it should be borne in 

 mind that Mr. Jones's observations prove that the zodiacal light 

 extends to a distance from the sun greater than the radius of 

 the earth's orbit. It is therefore an obvious inference from 

 these considerations, to ascribe the zodiacal light to the gravity - 

 undulations from the sun, and the luminous arch to those from 

 the earth. 



The reason that the terrestrial undulations produce a sensible 

 effect in regions where the effect of the solar undulations is too 

 faint to be perceptible is, that they are really much the more 

 powerful within a certain distance from the earth. The distance 

 from the earth, along the prolongation of the earth's radius- 

 vector, at which the effects would be equal, may be calculated 



