Prof. Challis on a Theory of the Zodiacal Light. 187 



on the following principle. By knowing the law of gravity and 

 the proportion of the attractions of the sun and earth at the 

 unit of distance from their centres, we can find the point on the 

 prolongation of the earth's radius-vector at which the sun's 

 attraction is just equal to that of the earth. This point will be 

 found to be at the distance of 152,500 miles nearly from the 

 earth's centre. Now, without borrowing from theory more than 

 that the gravitating effect of sethereal undulations, and their 

 luminous effect now under consideration, each depend on terms 

 of the order of the square of the velocity, we may, I think, pre- 

 sume that, since in each case there is no other difference of 

 circumstances than that one set of undulations proceeds from 

 the sun and the other from the earth, the effects of both kinds 

 are in the same proportion. Accordingly, at distances from the 

 earth within that above stated, the luminous effect of the ter- 

 restrial undulations will exceed that of the solar. It is there- 

 fore reasonable to infer that the total effect of the coexistence 

 within that space of the terrestrial and solar undulations and 

 the solar gyrations is the appearance of the luminous arch. It 

 is evident that it may present the considerable breadth of 20° 

 from its having its origin at points comparatively near the earth, 

 and that it is at the same time limited in breadth on account of 

 the limited transverse extent of the gyrations. 



The «above explanation seems to be open to the following 

 objection. It may be urged that the earth's rotation about its 

 axis must also produce gyratory motions of the sether, and that 

 the coexistence of these with the terrestrial gravity-undulations 

 should, according to the same principles, produce a luminosity 

 disposed about the plane of the earth's equator, presenting the 

 appearance of a luminous belt in that direction. To this I 

 reply that, judging from the phenomena of the zodiacal light 

 where it approaches the body of the sun, the luminosity would 

 be spread over the whole of the earth's surface, and, without 

 presenting a definite outline, would only be somewhat in excess 

 as seen from the equatorial parts. In fact, in this way we may 

 account, in great part, at least, for the luminousness of the sky 

 which is observable every clear night at all places and all seasons 

 of the year. This phenomenon has not escaped the attention of 

 either theorists or observers. Mr. Jones has made the following 

 interesting statement of what he noticed at Quito. " The thin- 

 ness of the air on this plateau, by which, during the day, objects 

 a great way off arc seen with remarkable distinctness, and seem 

 to be near by, might be expected to assist in atmospheric deve- 

 lopments of several kinds, unusual at other places; and such is 

 also the fact. A phenomenon which has sometimes drawn the 

 attention of philosophers, namely, a general brightness of the 



