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



air at night, without any apparent cause for it, is in some nights 

 very remarkable here. I find it has been noticed by others as 

 well as myself. The brightness, without moon or any assign- 

 able reason, has been so great at times, that I have taken out 

 printed papers or books to ascertain whether I could not see to 

 read by it." 



With respect to the variations of the visibility of the arch in 

 different seasons of the year, noticed by Prof. Brorsen, two cir- 

 cumstances have to be considered. First, as in the case of the 

 zodiacal light, the visibility of so faint a luminosity is dependent 

 on the inclination of the ecliptic to the horizon at the times of 

 observation. It seems that on this account the observations at 

 Senftenberg were chiefly made near the times of the vernal and 

 autumnal equinoxes. Again, the plane of symmetry of the 

 zodiacal light being inclined at a small angle to the plane of the 

 ecliptic, and the arch having its origin at the outlying portions 

 of the gyrations, the degree of its brightness will depend, much 

 more than that of the zodiacal light, on the earth's proximity to 

 the nodes. There being reasons for concluding, as I showed in 

 the former paper, that the earth passes the nodes about June 

 6 and December 6, it will be seen why, as stated by Prof. 

 Brorsen, the brightness increases on advancing from the equi- 

 noxes towards those epochs, and why, regard being had to the 

 positions of the ecliptic, the times most favourable for its visi- 

 bility are not quite synchronous with those for the visibility of 

 the zodiacal light. 



Another fact distinctly attested by Prof. Brorsen, namely, the 

 greater visibility of the arch in spring than in autumn, seems 

 to point to some degree of eccentricity of the zodiacal light rela- 

 tive to the sun's position. An eccentricity of this kind might 

 very reasonably be attributed to an effect of the disturbance of 

 the aether caused by the motion of the body of the sun in space. 

 Hitherto we have taken no account of this disturbance; but it 

 is evident that the motion of translation produces, as well as the 

 rotatory motion, a steady motion relative to the sun, and that 

 the whole relative steady motion of the sether is really com- 

 pounded of the effects of the two movements. The resolved 

 part of the sun's motion perpendicular to the plane of symmetry 

 of the zodiacal light would have no effect in producing the 

 eccentricity; but the part resolved in that plane would, by the 

 confluence of the relative streams which pass by or through the 

 sun, produce an elongation of the disturbance, or a kind of 

 wake, in the quarter from which the sun is moving. Thus the 

 motions concerned in generating the luminous arch would not 

 be the same in all directions from the sun. I shall not pursue 

 this part of the subject further at present, because it will pro- 



