154 PROCEEDINGS OF THE AMERICAN ACADEMY 



the elongation 90°. It is also apparent that the small mean value 

 here obtained for the "latitude of the axis" results from the extension 

 of the " Diffuse " Light, near its vertex, towards the south, which 

 Jones repeatedly observed towards the end of October, 1853. In 

 order to decide whether this extension has any bearing upon the 

 questions now before us, the work of Jones must be consulted. Con- 

 sidering the probable diinculty of observing the "Diffuse" Light in 

 the neighborhood of the Milky AVay, it does not seem to me that we 

 are warranted in attaching much significance to the location, during a 

 single fortnight, of the small part of its southern boundary wiiich 

 could be laid down upon the charts. 



The observations of zodiacal light at elongations of 120^^ or more 

 arc too few for profitable discussion in the inquiry immediately before 

 us. In what follows, I shall assume that the zodiacal light shifts its 

 place in latitude according to the changing position of the ecli[)tic in 

 the visible hemisphere. To decide whether this change can reason- 

 ably be explained by atmospheric absorption will demand some infor- 

 mation respecting the brightness of the zodiacal light, the relative 

 brightness of its different parts, and the nature of the lines considered 

 as its boundaries by different observers. No precise knowledge has 

 yet been gained upon these subjects, but we are not entirely in igno- 

 rance with regard to them. 



The general brightness of the zodiacal light has been usually esti- 

 mated by comparisons made with the Milky Way. Humboldt found 

 it equal at times to the region of Sagittarius (Hb. 120). Weber 

 repeatedly noticed that the zodiacal light was brighter than the Milky 

 Way, and on one occasion nearly twice as bright as its brightest por- 

 tions, (1874, March 5, II. 52; see also H. 37, 40, 43, 54). Eylert 

 makes very similar estimates (IL 47, 49). In Schmidt's general 

 description of the light (S. 15), he says that its brighter portions, 

 under favorable conditions for observation, exceed the mean brightness 

 of the Milky Wa}'. On January 26, 1848, and on Miivch 9, 1850 

 (S. 31, 38,) he found the zodiacal liglit brighter than the brightest 

 parts of the Milky Way. He communicated to Heis (H. 27) a more 

 precise observation made at Athens in April. 1802, when the lower 

 and middle part of the light a])peared to be from five to six times as 

 bright as the parts of the INIilky Way at the same altitude in Argo 

 and Cepheus. Jones occasionally describes portions of the zodiacal 

 light as equally bright with specified regions of the IMilky Way (Jj. 

 150, 152, 16G), and remarks the simultaneous appearance of both 

 objects, March 25, 1854 (Jj. 258). On two occasions he mentions 



