156 PROCEEDINGS OP THE AMERICAN ACADEMY 



Heis and TVeber probably attempt to include all the visible light 

 within the boundaries which they adopt. This gives them a zodiacal 

 cone wider at G0° than that of Schmidt. The " Stronger " Light of 

 Jones is of an intermediate width ; his "Diffuse" Light extends much 

 farther than any other observer has carried the limits of the light. 

 The form of the various cones or columns thus produced may require 

 separate consideration at some other time ; for the present, we have 

 merely to notice the arbitrary character of the boundary. It appears 

 to me a fair inference that the light fades away very gradually towards 

 its edges, and this conclusion will be approved, I think, by most observ- 

 ers, as one accordant with their general impression of the appearance 

 of the zodiacal light. Other occupations have always prevented me 

 from making any careful comparisons of the kind required to decide 

 the question ; but so far as I can judge from casual observation, the 

 light fades out as above described. 



The effect produced by the zodiacal light in dimming the light of 

 stars has apparently been seldom observed ; at all events, the few 

 results of such work which I have found on record are not sufficient to 

 permit any inference except that the light is too fiiint to interfere 

 materially with the perception of even a small star. Observations 

 of this kind were proposed by Argelander (A. lo6), but must be very 

 numerous and frequent to make them of value. 



Since the Milky "Way is at present our only standard of comparison 

 for the zodiacal light, it is desirable to form some idea of its bright- 

 ness. Our chief resource must probably be the results obtained by 

 the star-gauging observations of W. Ilerschel and others. The follow- 

 ing ratios of the maximum and minimum density of the stars found in 

 different regions of the sky appear in Celoria's discussion (C. 45) : — 



Uranometria Nova, Zone 0° to -\-(J° 5.15 



" _ioo '« -f 16° 6.29 



Durchmusterung, to magn. 7.5 3.94 



8.0 2.90 



9.5 4.20 



Milan observations ,3,04 



Herschel's gauges G2 78 



The manner in which these ratios were obtained is fully explained 

 in the work of Celoria. Althouijh the remons considered are of 

 limited extent in declination, they are sufficiently large to entitle the 

 results derived from them to much confidence. The regions of maxi- 

 mum stellar density are well known to be in the Milky Way, and it is 

 needless here to repeat the evidence of this fact. The Milan observa- 



