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40 COSMOS.is manifested during the twenty-five days immedia'.ely pro»ceding and succeeding the comet's perihelion passage. Thevalue of the constant is therefore somewhat different, becausein the neighborhood of the sun the highly attenuated butstill gravitating strata of the resisting fluid -are denser. Gibersmaintained"^ that this fluid could not be at rest, butmust rotate directly round the sun, and therefore the resistanceoffered to retrograde comets, like Halley's, must differwholly from that opposed to those comets having a directcourse, like Encke's. The perturbations of comets havinglong periods of revolution; and the difference of their magnitudes and sizes, complicate the results, and render it dilf?-cult to determine what is ascribable to individual forces.The gaseous matter constituting the belt of the zodiacallight may, as Sir John Herschelf expresses it, be merely thedenser portion of this comet-resisting medium. Although itmay be shown that all nebulae are crowded stellar masses,indistinctly visible, it is certain that innumerable comets fillthe regions of space with matter through the evaporation oftheir tails, some of which have a length of 56,000,000 ofmiles. Arago has ingeniously shown, on optical grounds, $that the variable stars which always exhibit white lightwithout any change of color in their periodical phases, mightafford a means of determining the superior hmit of the densityto be assunaed for cosmic al ether, if we supposeit to beequal to gaseous terrestrial fluids in itspower of refraction.The question of the existence of an ethereal fluid fillingthe regions of space is closely connected with one warmlyagitated by "Wollaston,^ in reference to the definite limit ofthe atmosphere— a limit which must necessarily exist at theelevation where the specific elasticity of the air is equipoisedby the force of gravity. Faraday's ingenious experiments on* Olbers, in Schum., Asir. NacJir., No. 268, s. 58.t Outlines of Astronomy , ^ 55G, 597.X ^^ En assimilant la watiere tres rare qui rempllt les espaces cilesteiquant a ses proprietes refringcntes aux gas terrestres, la density de cettimatiere nz saurait depasscr nne certaine limite dont les observations deeiloilcs chcngeantes, p. e. cellcs d'' Algol ou de (3 de Persic, peuvent assignerla valeur?^— Arago, in the Annnaire pour 1842, p. 336-345." On comparing the extremely rare matter occupying the regions oif space withterrestrial gases, in respect to its refractive properties, we shall find thatthe density of this matter can not exceed a definite limit, whose valuemay be obtained from observations of variable stars, as, for instance,Algol or (3 Tersei."i See WoWaRton, Philos. Transact, for 1822. p 80' Sir.Tnlin TIerschelcyp. cU., ^M, 36.