170 COSMOS, 



in another place,* 7 and which are not devoid of interest in our 

 endeavours to distinguish the different periods of development 

 which have characterised the history of astronomy. 



Before we pass from the very ancient, but artificial zodiacal 

 grouping of the fixed stars, as regards their sujlposed inser- 

 tion into solid spheres, to their natural and actual arrangement, 

 and to the known laws of their relative distribution, it will be 

 necessary more fully to consider some of the sensuous pheno- 

 mena of the individual cosmical bodies their extending 

 rays, their apparent, spurious disc, and their differences of 

 colour. In the note referring to the invisibility of Jupiter's 

 satellites, 38 I have already spoken of the influence of the 

 so-called tails of the stars, which vary in number, position, 

 and length in different individuals. Indistinctness of vision 

 (la vue indistincte) arises from numerous organic causes, 

 depending on aberration of the sphericity of the eye, diffraction 

 at the margins of the pupil, or at the eye-lashes, and on the 

 more or less widely diffused irritability of the retina from the 

 excited point. 89 I see very regularly eight rays at angles of 45 



clined, however, at an earlier period, to the idea of a solid icy 

 vault of heaven congealed from the absence of solar heat: 

 " Orbis ex aqua factus gelu concreta propter solis absentiam." 

 (Kepler, Epit. Astr. Copern., i. 2, p. 51.) Two thousand years 

 before Kepler, Empedocles maintained that the fixed stars 

 were rivetted to the crystal heavens, but that the planets w r ere 

 free and unrestrained" (TOVS Se irXavrjTas aveia-Gai]. (Plut. plac. 

 phil., ii. 13 ; Emped. 1, p. 335 Sturz; Euseb. Prap. evang., xv. 

 30, col. 1688, p. 839.) It is difficult to conceive how, ac- 

 cording to Plato in the Timaeus (Tim., p. 40, B., see Bonn's 

 edition of Plato, vol. ii. p. 344, but not according to Aris- 

 totle,) the fixed stars, rivetted as they are to solid spheres, 

 could rotate independently. w Cosmos, vol. ii. pp. 696-7. 



38 Vide supra, p. 64, and note. 



* " Les principales causes de la vue indistincte sont : aber- 

 ration de sphericite de 1'ceil, diffraction sur les bords dela pu- 

 pille, communication d'irritabilite a des points voisins sur la 



