136 THE APPARENT POSITION OF THE ZODIACAL LIGHT. 



The first definite discovery tending to account fi:)r the perplexing discordances 

 above mentioned, in the results of observation, was made by Jones,^ who found 

 that the apparent position of the light varied, in general, with the inclination of 

 the ecliptic to the horizon. In the article already mentioned, I stated, as the 

 apparent result of the data there collected, that this law is confirmed by the expe- 

 rience of other observers as well as by that of Jones, and that it should be re- 

 ferred to the effect of atmospheric absorption, as has been suggested by Geelrauyden. 

 A further inference is that the method of observation by drawing outlines, as here- 

 tofore practised, is insufficient, and that it must be replaced by photometric 

 observation of some sort. 



It may be practicable, however, to obtain from the older observations some 

 suggestion with regard to the apparent position of the zodiacal fight, after cor- 

 recting them roughly for the presumed effect of atmospheric absorption. Any 

 sugarestion thus attainable will lend additional interest to the work of future 

 observers, although it can have no great importance of its own. In undertaking 

 the inquiry, we shall be obliged to limit ourselves to the comparison of observa- 

 tions made by the same observer, since we already know that different observers 

 form very different conceptions with x-egard to the outfines of the zodiacal light. 

 It will also be desirable to compare observations made at the same elongation 

 from the Sun, but at points differing considerably in altitude while they differ 

 little in longitude. We likewise require observations made in as many different 

 parts of the zodiac as possible. No published observations except those of Jones 

 fulfil these conditions §ven approximately, and the inquiry here undertaken is 

 therefore limited to the work of that observer. 



The selection of observations for discussion was determined by the joint con- 

 sideration of the time required by the reductions and the probable interest of 

 the results to be derived from them. Upon the whole, it seemed best to employ 

 all observations of what was called by Jones the " Stronger Light," made either 

 in the evening or in the morning at the elongation 60^ and to omit the re- 

 mainder. The limits of the light at this elongation are given for the evening 

 observations in my former communication. Table X.^ In collecting and reducing 

 them for the present purpose, the following corrections were noticed to be required 

 by that table. Under the date of Nov. 4, 1853, column "Elongation of Vertex," 

 for the "Stronger Light," the figures 79, 82, 94, should be 80, 85, 96; for the 

 "Diff"ase Light," 106, 110, 114, 120 should be 109, 113, 117, 124; the data for 



1 Observatiotts on the Zodiacal Liglit, Introduction, p. xvi. ^ proc. Am. Acad. XIX. 183-191. 



