Prof. Challis on a Theory of the Zodiacal Light. 185 



In a subsequent communication, entitled " On the Ring-form 

 Shape of the Zodiacal Light/' and dated October 18, 1858, Prof. 

 Brorsen gives observations made at Senftenberg from April 1854 

 to October 1858, of the positions of maximum intensity of "the 

 faint extension of the ordinary zodiacal light from west and east 

 along the ecliptic," which he had described in his former letter. 

 The observations are all made either in February, March, April, 

 and May, or in August, September,and October; and certainly the 

 estimated positions of the maximums agree remarkably with the 

 points of opposition to the sun. The second letter is contained 

 in the Astronomische Nachrichten, vol. xlix. col. 219. 



Allowance being made for the difference of positions of the 

 observers, it can hardly be doubted that these two accounts refer 

 to the same phenomenon. The chief point of discrepancy is that 

 the American observer does not notice a position of maximum 

 intensity, but states, on the contrary, that the luminous arch is 

 uniform quite across the sky, while on this fact the other ob- 

 server speaks very decidedly. On the other hand, it may be re- 

 marked that Prof. Brorsen's description of the phenomenon in 

 the first half of November, as being an extremely faint girdle of 

 light extending along the ecliptic to the horizon, agrees well 

 enough with the result of the observations at Quito, the differ- 

 ent estimations of the intensity of the light being readily ac- 

 counted for by the difference of the localities. 



Such being the account of the principal features of the phe- 

 nomenon, I proceed now to offer a theoretical explanation of it, 

 after recalling attention to the theory of the zodiacal light pro- 

 posed in my last communication. The latter luminosity is there 

 ascribed to gyrations of the sethereal medium produced by the 

 rotation of the sun about its axis, and to disturbances which 

 they undergo by reason of the motion of translation of the 

 sun in space. It was considered that the effect of this motion 

 would be the same as if the aether were impressed with an 

 equal motion in the opposite direction. The sun would thus be 

 at rest, and together with the steady gyrations there would be a 

 uniform stream of the aether. It was proved by mathematical 

 reasoning that such steady motions might coexist without mutual 

 disturbance, so far as terms including the square of the velocity 

 indicate, and that disturbances would arise out of motions 

 expressed by terms of the third and higher orders relative to the 

 velocity. Such disturbances, it was supposed, might become 

 sensible in the form of light-undulations originating in the space 

 common to the two motions. It is, however, to be said that in 

 this way the zodiacal light may be accounted for, but no expla- 

 nation is given of the related phenomenon of the luminous arch ; 

 and, moreover, it must be admitted that no reason is given in 



