162 PROCEEDINGS OF THE AMERICAN ACADEMY 



Any considerable series of photometric observations of the zodiacal 

 light, made with trustwortliy apparatus and upon a suitable system, 

 would doubtless furnish better means of determining the real position 

 of the light with regard to the ecliptic than could be provided by the 

 most voluminous collection of sketches. We should thus acquire some 

 reliable information about the inclination and nodes of the great circle 

 representing the axis of the light, which has hitherto been sought with 

 little success. But our work, to be complete, must include an inquiry 

 into the normal distribution of light in the sky. The zodiacal light 

 is seen upon a background by no means of uniform brightness, even 

 in regions remote from the Milky Way. Its more brilliant portions, 

 very probably, may not be sensibly affected by the inequalities of the 

 background, but its edges, and especially its vertex, cannot be deter- 

 mined without attention to these inequalities. The interest of a care- 

 ful photometric inquiry into the relative light of different parts of the 

 sky, and especially into the exact form of the Milky Way, would be 

 considerable, even without reference to its bearing upon the question 

 of the zodiacal light. But the inquiry is indispensable if we are to 

 substitute definite knowledge for the vague information now before 

 us with regard to " zodiacal bands," the singular phenomenon of 

 " Gegenschein," and the possibly periodical variations in the main 

 body of the zodiacal light, as well as its a[)parent changes from hour 

 to hour. 



As a first instance, let us consider the semiannual variation in the 

 elongation of the vertex, evidence of which Serpieri has obtained from 

 the observations of Jones (Sp. 99), confirming the result by similar 

 data from the work of Heis (Sp. 101, H. 60). A comparison of the 

 mean elongations for the successive months given by Serpieri, from 

 his reduction of the work of Jones, with the corresponding series in 

 Table XIII. of the Appendix, shows a sufficient general agreement, 

 althougli there was a considerable difference between the two methods 

 of reduction, and even in the processes by which the separate elonga- 

 tions were derived from the observer's charts (Sp. G2). 



In both cases, we have two yearly maxima of elongation (about the 

 beginning and middle of each calendar year) with a tolerably regu- 

 lar diminution and increase from each maximum to the next. The 

 "Stronger" Light, however, shows no maximum for the evening 

 observations of the beginning of 1854, nor, according to Serpieri, for 

 the morning observations of the middle of 1853. 



Before concluding that these variations indicate any important 

 annual series of changes in the zodiacal light itself, we must examine 



