OP ARTS AND SCIENCES. 155 



the superior brightness of the zodiacal light (Jj. 134, 298). The 

 luminous arch which he afterwards observed at Quito was approxi- 

 mately equal to the Milky Way in brightness, but on the whole seems 

 to have been sh'ghtly inferior to it (Jq. 377). Gruey's observations 

 at Toulouse in February, 1875 (T. 905), make the zodiacal liglit 

 about as bright as the Milky Way ; sometimes a little brighter or 

 fainter. See also the account of Lewis (Ls. 439). From all these 

 remarks it appears pretty certain that the zodiacal light and the 

 Milky Way are objects of the same general order of brightness, so 

 that if the light of equal areas of their brightest portions is supposed 

 to be concentrated at single points, the stars so formed would not 

 differ by more than two magnitudes, and would probably differ by less 

 than one magnitude. 



The relative intensity of different parts of the zodiacal light seems 

 not to have been investigated. We have reason to think, however, 

 that the light fades out by degrees as it extends farther from the Sun, 

 and also as it extends laterally from its medial line, or " axis." This 

 appears more or less distinctly in all accounts of the phenomenon, but 

 for the most part only indirectly, by the difficulty experienced in 

 tracing the boundaries of the light, especially towards the vertex. 

 An object as bright as many well-marked parts of the Milky Way, 

 and of nearly uniform brilliancy, would of course be located without 

 trouble. But there is also positive testimony to the same effect. 

 Schmidt tells us (S. 15) that the light increases towards the horizon 

 until checked by the denser strata of the atmosphere, and also men- 

 tions (S. Gl) the comparative indistinctness of the vertex and the 

 unequal intensity of different parts of the light. Jones makes a sim- 

 ilar statement in his Introduction (Jj.xi). He also says (Jj. 346) 

 that the " Diffuse " Light " faded away imperceptibly at its edges " ; 

 and that the " Stronger " Light " dims as it ascends." The lower por- 

 tions were quite distinct on another occasion (Jj. 376) before the night 

 had deepened sufficiently to show the remainder. The general descrip- 

 tion by Lewis (Ls. 438) confirms the preceding accounts. 



The nature of the boundaries adopted by different observers appar- 

 ently differs very considerably. In the case of Schmidt, we see by 

 his account (S. 23) that the boundary is a line within the extreme 

 limits of the light, selected as one which can be followed among the 

 stars with a certain degree of confidence. The comparative narrow- 

 ness of the light recorded by this observer, which will appear for the 

 elongation 00° on inspection of Table I., is in conformity with the 

 method of observation just described. 



