VI INTRODUCTION. 



sphere, and is, therefore, capable of emitting dense vapors, and, moreover, 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. The dia- 

 gram which is attached to his article on this subject in the Memoires de I'Acadcmie Boyale des 

 Sciences, vol. 8, gives the shape of this body or emanation, as lenticular, with its greatest 

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

 seen by us in June ; at which latter time, also, it is drawn so as not to reach the horizon of a 

 spectator in the latitude of Paris. Whether he meant to have this lenticular-shaped medium to 

 be considered as attached to the sun, and revolving at the same time with it, or not, is not appa- 

 rent from his writings. At one time he appeared to think that this matter might consist of an 

 infinite number of small stars, such as are thought to give us the light of the Milky Way, and 

 he made telescopic observations with reference to such an idea; but the results were not satis- 

 factory to his mind. So, also, with scintillations or bright dartings of brief light along the 

 course of the Zodiacal Light, which for a while were thought to be seen; further observations 

 made these doubtful, and the idea of such appearances was laid aside. 



I have dwelt so long upon Cassini's observations and theories, inasmuch as these obser- 

 vations, though made one hundred and seventy years ago, exceed in amount all others 

 subsequently printed ; and his annotations have a distinctness which I have not been able to 

 find in any other work ; besides that his theory has had such an important influence 

 over the minds of astronomers in all subsequent times. He continued his observations, 

 pretty regularly, for six years; and afterwards in a more desultory manner, till 1693; and 

 thus we have eleven years of very important records from his pen. I have, from his detailed 

 descriptions, been able to draw outlines on my star-charts for eleven nights, when his lati- 

 tudes and my own were sufficiently near for comparison of results ; and I find that, in many 

 instances the boundaries of the Zodiacal Light, at his time (one hundred and seventy years 

 ago), were almost, or perhaps quite, the same as at present. It is somewhat difficult to get his 

 boundaries precisely, inasmuch as he describes them by the figures of the pictured constellations ; 

 which pictures, we know, are often varied to suit the fancy of the artist dealing with the stars, 

 and are therefore no certain guide. Some of these outlines from his records, drawn out on 

 charts, are appended to this work (Nos. 342-351) for comparison with those of our own 

 times. 



Cassini's labors gave a start to other observers, among whom the most eminent was Fatio de 

 Duillier, for a while his colleague at Paris, but afterwards residing near Geneva, where he 

 made observations; of which, however, we have only a few records in Cassini's own work. 

 Fatio de Duillier, however, is worthy of particular notice, as having, instead of Cassini's idea 

 of reflecting particles originating in the sun, conceived that of particles of matter distinct from 

 the sun, and arranged in shape like a "solid zodiac;" which body of uneven surfaces, and rota- 

 ting around the sun, he supposed to give us the Zodiacal Light.* Observations were also made 

 by Kirch and Eimmart, in Germany, from 1688 to 1694, and were published, but only a few of 

 them in detail. 



" II pi. Fatio de Duillier] me communiqna [7 October, 168-1] I'bypntliese qu'il avait coneiie six on sept innis aupar.i- 

 vnnt. Elle a cela de comun avec ce quej'avais propose" dans le Journal du 1C83, qu'il suppose dans IVUu-r des particulcs 

 capaliles de dcjtuurner et de re'fleehir Inminv. II Irs dispnsa hint aiitour dn soldi cmnme dans un zodiaque solide, large, et 

 inv.nnlicr, cmnpris entre deux surfaces oourbes et ondoyantcs, en sortr qn'elles pnissenl comprcndiv dans un nioindre espace 

 les oH'itcs 'li's [ilauetcs de'crites autour du snk-il, placces a divcrscs distances et iiieliiu'es diverseinent 1'une vors 1'autre. Le 



