120 Trof. Challis on the Zodiacal Light. 



light are coincident with the nodes of the sun's equator. In the 

 same case the maximum apparent inclinations would occur at 

 June 6 and December 6, which, as will presently be shown, is in 

 sufficient accordance with the results of observation ; while, ac- 

 cording to M. Houzeau's elements, about seventeen days before 

 those epochs the inclinations are at zero. 



In order to test the above inferences, I have calculated the 

 apparent inclinations from as many observations as I could col- 

 lect of positions of the apex taken within moderate intervals from 

 June 6 and December 6, and have compared them with the 

 inclinations, deduced in the manner above stated, from the ele- 

 ments of the sun's equator. The observations used for this 

 purpose are fourteen in M. Houzeau's list from October 26 to 

 January 8 inclusive; two taken by Prof. Smyth on July 9, 1845 

 (those of June 5, which for some reason are wholly discordant, 

 being omitted), and five taken by Capt. W. S. Jacob in 1862, 

 on May 17 and 26, and June 11, 16, and 21. These last are con- 

 tained in the ( Monthly Notices of the Royal Astronomical 

 Society' (vol. xxiii. No. 2, p. 55), in a letter from the observer 

 communicated by Prof. Smyth, who has added concluded right 

 ascensions and declinations of the apex, which I have adopted. 

 Of the four remaining observations by Capt. Jacob, those of July 

 16, 17, and 22 were thought to be too uncertain, and that of 

 July 30 taken advisedly when Venus was behind a cloud, and 

 giving a consistent result, was considered to be too distant from 

 the epoch of June 6. The mean of the inclinations given by the 

 twenty-one observations is 4° 16', and the mean of those deduced 

 from the elements of the sun's equator 6° 29'. The difference, 

 2° 13', being subtracted from 7° 9', the assumed inclination of 

 the solar equator, the remainder, 4° 56', may be regarded as the 

 apparent inclination at the epoch of December 6, and, in fact, 

 the maximum apparent inclination, very approximately deduced 

 from the observations, if the nodes are coincident with those of 

 the sun's equator. The apparent inclination at the same epoch 

 by the other elements is 0°54', and the maximum apparent incli- 

 nation, occurring at September 25, is 3° 35'. The above differ- 

 ence of 2° 13' may be attributable partly to the rough and un- 

 certain character of the observations, partly to the error of the 

 hypothesis of the calculations, and partly to error in the determi- 

 nation of the inclination of the solar equator. I take the occa- 

 sion to remark that the measures of the positions of solar spots by 

 which this determination has been made, do not take account of 

 any displacement of those positions which may be due to the 

 passage of the rays by which the spots are seen through the 

 sun's atmosphere. 



Upon the whole, it may be concluded from the above discus- 



