THE APPARENT POSITION OF THE ZODIACAL LIGHT. 147 



Barnard.^ It so happens that those who see the phenomenon best have been 

 prevented by other occupations from observing it frequently and with precision, 

 or at least from publishing in detail, and in some generally accessible place, the 

 observations which they may have made. But in view of the triple discovery of 

 " Gegenschein," and of the considerable, though fragmentary, mass of published 

 observations respecting it, little doubt can be felt of its reality. Assuming it to 

 exist, the probability that it is part of the zodiacal light is certainly strong, 

 although there is no easy theoretical explanation of its appearance. The readiest 

 way of accounting for it, on the meteoric theory of the zodiacal light, would be 

 to assume such a law for the phases of the meteors that their brightness would 

 rapidly increase as they approached opposition. Some indications of the possi- 

 bility of such an increase have been given in a former communication.^ A less 

 natural explanation might be sought in the possible perturbations of meteoric 

 matter by the Earth .^ 



The conclusion that the zodiacal light lies farther to the north near the 

 autumnal than near the vernal equinox may accordingly be regarded as consid- 

 erably strengthened by the agreement between the observations of " Gegenschein " 

 and those made by Jones of the brighter portions of the zodiacal light. In any 

 future observations of either kind, it will be interesting to notice whether further 

 evidence in support of this conclusion is obtained. Perhaps it will also be possible 

 to define the position of the light more accurately than could be done by the 

 older methods of observation, and to trace any progressive changes which may 

 occur in it. 



The main hindi'ance to the development of the meteoric theory of the zodiacal 

 light is the want of trustworthy information with regard to the probable phases 

 of the meteors. Writers who have attempted to treat the subject mathemati- 

 cally* have hitherto contented themselves with Lambert's formula, sin v — v cos v, 

 which has a very imperfect foundation in experiment, and does not apply to the 

 phases of rough bodies, even if the hypotheses on which it rests are correct. 

 Experiments are now in progress, however, at the observatories of Munich^ and 

 of Harvard College, which may add considerably to our knowledge of the laws 

 of irregular reflection. After enough information of this kind has been collected, 



1 Sidereal Messenger for November, 1883, II. 254. ' Proceedings Am. Acad., XIX. 310. 



' Astronomische Nachrichten, CII. 266. 



♦ Schiaparelli, Entwurf einer astronomischen Theorie der Steraschnuppen (Gennan translation by Boguslawski), p. 194; 

 Geelmuyden, Reniarques sur la theorie de la lumi^re zodiacale, p. 104. 



* Vierteljahrsschrift der Astronomischen Gesellschaft, XX. 111. 



