Prof. Challis on the Zodiacal Light, 123 



the zodiacal light depends on the following hypotheses, which are 

 the same that I have previously made for the explanation of facts 

 of a very different kind. First, I assume the existence of the 

 aether, that is, of a uniform elastic fluid pervading space and 

 pressing always proportionally to its density. Secondly, the 

 aether is assumed to be the medium of the generation and trans- 

 mission of light, this assumption being abundantly justified by 

 the explanations deducible from it of a great variety of pheno- 

 mena of light, as is shown in the Number of the Philosophical 

 Magazine for last December. Thirdly, the sether is supposed to 

 permeate freely all visible and tangible substances, filling without 

 change of its density all the space not occupied by the atoms of 

 the substances. 



It is an ascertained fact that the vast body of the sun revolves 

 uniformly about an axis in twenty-five days nearly. Without 

 inquiring into the particular disturbing effect of this movement 

 on the sether, we may with certainty draw from the above hypo- 

 theses the following inferences, which are all that are necessary 

 for my present purpose : (1) that as the disturbing cause is con- 

 stant, the motion of the aether produced by it is steady ; (2) that 

 the motion decreases with increase of distance from the sun, and 

 with increase of distance from a plane coincident with that of 

 the sun's equator; (3) that the motion is symmetrical with 

 respect to the sun as its centre, and is alike on the two sides of 

 the above-mentioned plane. These inferences are true if the 

 sun's centre have a fixed position in space. But there is good 

 reason to conclude, from astronomical observation, that the sun 

 and the whole solar system is progressing through space with a 

 uniform rectilinear motion. To take account of the effect of 

 this motion, we may conceive to be impressed on the aether 

 an equal contrary motion. The sun will thus be reduced to 

 rest, and we shall have the case of two coexistent steady motions 

 having independent origins. According to the hydrodynamical 

 law proved above, the steady motion caused by the sun's rotation 

 v/ill not be altered by this circumstance, and will consequently 

 be the same when the sun is in motion as if it were at rest. 



It is now important to explain in what manner the coexistence 

 of these steady motions gives rise to the appearance of light. 

 This is a theoretical consequence of the analytical circumstance 

 that, in the investigation of the law of coexisting steady motions, 

 terms of the third order were omitted. These terms correspond 

 to parts of the motion which, so far as they are effective, pro- 

 duce disturbances of the aether superadded to the steady motions ; 

 and the disturbances, since they arise from the mutual action of 

 the parts of the fluid, are productive of light-undulations, accord- 

 ing to the mathematical principles of the undulatory theory of 



