202 • COSMOS. 



sentdtioii of universal space, it is necessary to call to mind 

 the difference of the members of the solar system, which by 

 no means excludes similarity of origin and lasting depend- 

 ence upon the moving forces. 



Great as is the obscurity which still envelops the material 

 cause of the zodiacal light, still, however, with the mathe- 

 matical certainty that the solar atmosphere can not reach 

 beyond -^-^ of the distance of Mercury, the opinion supported 

 by Laplace, Schubert, Arago, Poisson, and Biot, according to 

 which the zodiacal light radiates from a vapory, flattened 

 ring, freely revolving in space between the orbits of Venus 

 and Mars, appears in the very deficient state of observation 

 to be the most satisfactory. The outermost limits of the Sun's 

 atmosphere, like that of Saturn (a subordinate system), could 

 only extend to that point where the attraction of the uni- 

 versal or partial central body exactly balanced the centrifu- 

 gal force ; beyond this point the atmosphere must escape at 

 a tangent, and continue its course either aggregated into 

 spherical planets and satellites, or, when not aggregated into 

 spheres, as solid and vaporous rings. From this point of 

 view the ring of the zodiacal light comes within the cate- 

 gory of planetary forms, which are subject to the universal 

 laws of formation. 



From the small progress which this neglected part of our 

 astronomical knowledge makes on the path of observation, I 

 have little to add to that which I derived from the experience 

 of others and myself, and have previously developed in the 

 Delineation of Nature (vol. i., p. 127-134 ; vol. iv., p. 308). 

 If, 22 years before Dominique Cassini, to whom the first de- 

 tection of the zodiacal light is erroneously ascribed, Chil- 

 drey, the chaplain of Lord Henry Somerset, had already re- 

 commended this phenomenon to the attention of astronomers 

 in his Britannica Baconica, published in 1661, as one which 

 had previously been unnoticed and observed by him during 

 several years, in February and the commencement of March, 

 so must I also mention (according to a remark of Olbers) a 

 letter which Rothmann wrote to Tycho, from whence it re- 

 sults that Tycho saw the zodiacal light as early as the end 

 of the sixteenth century, and considered it to be an abnormal 

 spring-evening twihght. The strikingly greater luminous in- 

 tensity of this phenomenon in Spain, upon the coasts of Va- 

 lencia and the plains of New Castile, first incited me to con 

 tinuous observation before I left Europe. The strength of 

 the light— it might almost be called illumination — increased 



