THE ZODIACAL LIGHT. 353 



Cassini discovered very soon that as time advanced through March 

 and April, the upper or northern edge of this light inclined more and 

 more from the ecliptic, and stretched farther to the northward ; and 

 knowing that the sun's equator, as shown by his spots, was also then 

 stretching off from the ecliptic in a similar way, he came to the con- 

 clusion that the substance giving this light was closely connected 

 with the sun's equator, and was consequently changing its position 

 with regard to that equator. 



He argued, further, that as the sun has an atmosphere, and is there- 

 fore capable of emitting dense vapors, and is continually sending out 

 matter of exceeding fineness which we call light, consequently this 

 luminary might also, by its motion on its axis, send out a substance 

 intermediate in character between the two ; which substance, either 

 self-luminous or by reflection, might give us the zodiacal light. To 

 support this theory, he gives to this body a lenticular shape, about 

 twice the thickness of the sun as seen in March, but only of the sun's 

 thickness when seen in June. Whether he meant to have this len- 

 ticular-shaped medium to be attached to the sun, and revolving with 

 it at the same time, is not apparent. He devoted a part of his time 

 for about eleven years, in a very desultory manner, to observing this 

 light. 



Cassini's labors led other observers to direct their attention to the 

 zodiacal light. Fabio de Duillier, who was his colleague for a while 

 at Paris, is worthy of particular notice, as having conceived the idea 

 that it consisted of particles of matter distinct from the sun, and 

 arranged in shape like a solid zodiac; which body of uneven sur- 

 faces, and rotating round the sun, he supposed, gave us the zodiacal 

 light. 



In 1731 Mairan gave considerable attention to this subject in a 

 work on "The Aui'ora Borealis." He advanced the theory that this 

 light is reflected from the sun's atmosphere, stretched out into a flat- 

 tened spheroid or lenticular-shaped body, revolving with the sun an 

 idea which Laplace has forever set at rest by demonstrating that the 

 sun's atmosphere can extend no farther than to the orbit of a planet 

 whose periodical revolution is performed in the same time as the sun's 

 rotary motion about its axis, or in twenty-five days and a half; that 

 is, only so far as nine-twentieths of Mercury's distance from the sun. 



Since the time of Mairan, until 1853, but little attention appears to 

 have been given to this subject. In 1833, however, Biot, in order to 

 account for the meteor-shower of that year, attempted to show that 

 the shower was owing to the earth's passing at that time near the 

 node of the zodiacal light. But calculations were made by J. C. 

 Housseau in order to see whether the nodes of the sun and the zo- 

 diacal light do actually correspond. After careful observations and 

 calculations, in which he was assisted by nine diligent observers, 

 Housseau thinks he has shown that these nodes are diflerent, and that 

 VOL. XI. 23 



