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Mr. U. A. JProctor on the Zodiacal Light. 



ties surrounding a subject of this sort assist us — in this way, 

 that they enable us to reject theories which otherwise might 

 engage our attention and so cause perplexity. Precisely as the 

 very complexity of a lock makes us all the more certain that a 

 key which opens the lock is the key really appertaining to it, 

 so, where a subject of astronomical research presents many per- 

 plexing phenomena, these become so many reasons the more for 

 accepting a theory which is not contradicted by any one of 

 them. 



This is, I think, the case with the zodiacal light. We are 

 able, as I hope now to show, by considering the peculiarities of 

 this object to get rid, one after another, of various theories which 

 might otherwise distract our attention. And though by this 

 process of elimination we may not be enabled to determine quite 

 the true theory of this object, we can yet considerably narrow 

 the field within which selection has to be made. 



The first considerations to be dealt with are those which de- 

 pend on the normal features of the zodiacal light. It is well 

 known that the light exhibits usually the figure of an oblique 

 conoid whose axis lies close by the ecliptic, and whose vertex 

 lies at a varying distance from the position of the sun. Near 

 the axis the light grows brighter, except close by the vertex, 

 where it is even fainter than at the other parts of the border. 

 The following Table, prepared by Herr Klein from modern ob- 

 servations, indicates the varying range of the vertex from the 

 place of the sun, though it must be remembered (and will be 

 recognized at once by every one familiar with the varying posi- 

 tion of the ecliptic during the year, and other like circum- 

 stances) that these measures indicate variation in the extent of 

 visibility rather than (of necessity) any real variation in the 

 extent of the light. 



Distance of vertex 

 Day of the year. from sun. Part observed. 



83-0 Western half. 



91-8 



81-0 



74-0 



75-0 



65-0 



77-0 



58-0 Eastern half. 



74-5 



71-3 



56-5 



61-0 



80-5 



January 2 





26 



February 11 

 March 14 





April 14 

 May 4 . 



August 1 

 September 15 

 October 17 





November 12 





29 . 

 December 13 





28 , 





