1G6 PROCEEDINGS OF THE AMERICAN ACADEMY 



connected with a band only about five degrees in width, and, more- 

 over, we might expect the maximum of elongation, if due to the 

 perception oi this band, to show itself earlier in the season. This is 

 actually the case in the observations of Dechevreus (D. 20), who fre- 

 quently noticed the band as an extension of the zodiacal light (D. 5, 

 10). It is natural that observers should not usually become aware 

 of this faint object until the season of the year when it has been 

 reached by tiie vertex of the ordinary zodiacal cone. In endeavoring 

 to find the vertex, the eye then follows the previously unnoticed band 

 as far as the Pleiades, without reaching any place at which the light 

 can be said to terminate. 



Before we can satisfy ourselves with regard to the true zodiacal 

 bands, as they may be called, which are described by Jones and 

 Lewis, it is necessary for us to observe accurately the extent and 

 brightness of the similar phenomena which have just been considered. 

 Moreover, our observations ought not to be confined to the zodiac, for, 

 if they are, we may overlook the existence of bands of light inter- 

 secting it at considerable angles, the zodiacal portions of which may 

 occasion perplexities in our subsequent work. The need of general 

 knowledge with respect to tiie normal distribution of light in the sky 

 tlms becomes evident, even if we seek only a thorough knowledge of 

 zodiacal phenomena. 



Admitting the occurrence of the phenomenon called " Gegenschein," 

 which certainly has a large mass of evidence in its favor, we must still 

 feel much uncertainty about its visibility on particular occasions, until 

 we are able to say with some confidence that the light we are observ- 

 ing is greater than the normal light of the region in wiiich it appears. 

 At present, we have only our memory of former observations to pre- 

 vent mistakes, and if the observer feels sure of the correctness of his 

 own memory, he ought to have some surer means of making the facts 

 of the case clear to others who have not witnessed them. For the 

 study of all zodiacal phenomena, therefore, a photometric survey of 

 the sky is urgently required. 



Although the o[)inion of Serpieri, that the zodiacal light is a species 

 of aurora (Sp. 153), will probably not find general acceptance in the 

 present state of our knowledge, it ini likely that auroral and zodiacal 

 light m ly sometimes be confounded with each other. Auroral bands 

 sometimes occur, as is well known, in the foi-m of arches extending 

 from the eastern to the western horizon, and retaining their form and 

 place for a considerable time. It is possible that such arches are 

 formed at times in the absence of any other auroral light. If this 



