IGO PROCEEDINGS OP THE AMERICAN ACADEMY 



tude, whether north or south, will be numerically less near the vertex 

 than near the base. Supposing this to be an effect of atmospheric 

 absorption, it appears that the increased brightness of the light at the 

 smaller elongations does not fully counterbalance the increased ten- 

 dency to a variation in latitude due to the increased difference in 

 atmospheric absorption nearer the horizon. This seems antecedently 

 probable, and indeed it may surprise us that we do not find much 

 greater variations in latitude at small elongations. In Table II., for 

 example, we have differences of absorption for the opposite bounda- 

 ries of the " Diffuse " Light, at the elongation 60°, amounting to two 

 or three entire magnitudes at the extreme inclinations of the ecliptic. 

 These differences would doubtless be lessened by a strict computation, 

 in which the position of the ecliptic would be computed separately for 

 the same observations employed to determine the place of the light in 

 latitude, instead of being found only for the mean of all the observa- 

 tions belonging to the group including them. However, in any case, 

 the resulting differences of absorption would be large ; and this would 

 likewise be true of similar differences computed at the elongation 30°, 

 either for the " Stronger " Light of Jones, or for the ordinary zodiacal 

 light of other observers. Another circumstance which may seem 

 inconsistent with the theory here advanced is, that when we compare 

 the results at the same elongation (G0°) for the " Stronger" Light in 

 Table I. with those for the " Diffuse " Light in Table II., we find 

 that a much smaller difference of absorption in the first case appears 

 to produce nearly as much change of latitude. Still, the change is 

 actually greater in the second case, and we have as yet no means of 

 determining its theoretical value in any instance. A light which is 

 actually confined to the zodiacal region cannot be shifted out of it by 

 any amount of atmospheric absorption, although it may be so far di- 

 minished as to cease to be perceptible. Again, we cannot be sure, as 

 has been said, that observers intend to signify equality of brightness 

 on opposite sides by the location of their assumed boundaries. If the 

 sky beyond the boundary is as much darkened as the sky within it, 

 which may happen in the absence of haze and of artificial light in the 

 neighborhood, the boundary might still be perceptible. Probably the 

 condition of equality of light on the opposite sides is partially, though 

 unconsciously, recognized in the effort to fix the boundaries, while at 

 the same time it is not regarded as essential, especially when the 

 boundaries are so far apart as those of the " Diffuse " Light at moder- 

 ate elongations. 



If atmospheric absorption has the importance here assigned to it in 



