60 



STATEMENT AND EXPOSITION OF 



With that in view, the special appearances of the Zodiacal Light may be arranged 

 as follows : — 



Case \st. The Zodiacal Light appears narrow and towering high just about the 

 time of the new moon ; a-s tJiough tlie sun's light tmre indeed 

 Fig, 15 transmitted, at that time, through the least curved, and, proba- 



bly, somewhat rarer sides of the oval-shaped girdle ; and that 

 through a great part of the length of the oval. {Fig. 16.) 



Case 2d. After the new moon, when the moon is approach- 

 ing her first quarter ; when the moon has set, and the twi- 

 I I light has disappeared, the Zodiacal Light does not extend so 



\ «/ high as in the preceding case, and its termination is broader, 



"^^^_^ ^^^ and not so sharply curved, and the intensity of the light, 



withal, is not especially conspicuous (as in Fig. 16, for Zodi- 

 acal Light of the morning), as though the sun's light indeed, in all its transmission, 

 passed through tlie ratlier less dense portion of the girdle ; 

 F'g- l*"^- and passed out of it in a direction more across the girdle 



and not so nearly at a tangent to it (in its exit under these 

 circumstances) , as in tlie preceding case. 



Case Zd. After the full moon, and when the moon is 



approaching her last quarter ; then, before the rising of the 



moon, and after the end of twilight, a luminous spot of 



\ / considerable size, and, in appearance, like the brighter por^ 



^^,^ .^^ tion of an aurora borealis, occupies the place in the Zodiacal 



Light which is quite accurately opposite to the moon's place ; 



and night after night, as the moon advances, this luminous spot rises among the 



stars, so as still to keep opposite to the moon ; as though the somewhat more dense 



portion at tlie farther end of the oval {as respects the moon) 



Fig. 17. tDere thus more conspicuous than the other portions then in 



vieio ; and then the upper extremity of the Zodiacal Light 



is broader and not so sharply pointed as in Case 1st; as 



f _ \ though for tlie reason assigned in Case 2d. {Fig. 17.) 



I _ } Case 4:ih. After the last quarter and before the new 



\ / moon, the Zodiacal Light of the evening is again faint, as it 



\® _J was before the first quarter ; ccs though the illumination loere 



"^^m^^_^,^ti^ wholly of that part of the girdle beyond the region near the 



longer axis. {Fig. 18.) 

 Case 5th. When the moon is nearly in quadrature, it Avould seem that the 

 Zodiacal Light must appear sho7't ajid bright, if apparent at all after the twilight 

 of the evening, or before the twilight of the morn-ing. For the sun's light would 

 be transmitted by a short course through the most curved portion, near to one end 

 of the longer axis of the oval. {Fig. 19.) 



(86) Increase of brightness might be looked for, with the moon. in perigee; and 

 of extent, with the moon in apogee. Traces of something like one and the other 

 have been apparent. 





