OP ARTS AND SCIENCES. 147 



of the vertex, and to tbe latitudes of the northern and southern boun- 

 daries at successive elongations 30° apart. The term " elongation " 

 will be used throughout this discussion to denote a difFerence of lon^i- 

 tude between the Sun and some other point, not the direct aucrular 

 distance of that point from the .Sun. Since the zodiacal lio-ht, how- 

 ever it may be produced, has plainly some relation both to the Sun 

 and also to some great circle not much inclined to the ecliptic, the 

 arrangement of the observations according to elongation (in the 

 sense just stated) appears to be the most natural which can be 

 adopted. The interval of SO"" is arbitrary. The knowledge of the 

 observations gained by reducing them now inclines me to think that 

 20° would have been preferable, although it would have increased the 

 labor of the work ; but I could not have the information necessary to 

 settle this question at the beginning of the reductions. 



In the following pages, I shall have occasion to cite a number of 

 works, the titles of which are here given, with the abbreviations after- 

 wards used to designate them. Figures following an abbreviation will 

 indicate the page of the corresponding work to which reference is 

 made. 



A. Argelander, Aufforderung an Freunde der Astronomie. (Pp. 122 

 -254 of Schumacher's Jahrbuch fiir 1844.) 



C. Celoria, Sopra alcuni Scandagli del Cielo. (Pubblicazioni del 



Reale Osservatorio di Brera in Milano, N. XIII.) Milano, 

 1877. 



D. Dechevrens, La Lumiere Zodiacale. Zi-Ka-Wei (China), 1879.- 

 G. Geelmuyden, Remarques sur la Theorie de la Lumiere Zodiacale. 



(Separataftryk af Archiv for Mathematik og Naturvidenskab.) 

 Kristiania (1882?). 



*H. Heis, Zodiacallicht-Beobachtungen in den letzten 29 Jahren 

 1847-1875. I. Veroffentlichung der konigl. Sternwarte zu 

 Miinster. Munster, 1875. 



Hb. Humboldt, Cosmos, Vol. I. (Bohn's Scientific Library.) Lon- 

 don, 1849. ' 



*Jj. Jones, Observations on the Zodiacal Light. (United States Ja- 

 pan Fxpedition, Vol. III.) Washington, 1856. 



Jq. Jones, Observations at Quito. (Am. Journal of Science for No- 

 vember, 1857, Vol. XXIV. pp. 374-385.) 



L. Loewy, Remarques sur la Methode pi'oposee par M. le Professeur 

 Pritchard pour la Mesure de I'Eclat des Astres. (Monthly 

 Notices of the Royal Astronomical Society, Vol. XLII. pp. 

 91-94.) 



