46 COSMOS. 



one, the less firmly based hypothesis, refers tc the limited 

 transparency of the celestial regions ; and the other, founded 

 on direct observation and yielding numerical results, is de- 

 duced from the regularly shortened periods of revolution of 

 Encke's comet. Olbers in Bremen, and, as Struve has ob- 

 served, Loys de Cheseaux at Geneva, eighty years earlier 81 

 drew attention to the dilemma, that since we could not con- 

 ceive any point in the infinite regions of space unoccupied 

 by a fixed star, i. e. a sun, the entire vault of heaven must 

 appear as luminous as our sun if light were transmitted to us 

 in perfect intensity; or, if such be not the case, we must 

 assume that light experiences a diminution of intensity in its 

 passage through space, this diminution being more exces- 

 sive than in the inverse ratio of the square of the dis- 

 tance. As we do not observe the whole heavens to be almost 

 uniformly illumined by such a radiance of light (a subject 

 considered by Halley 33 in an hypothesis which he subse- 

 quently rejected) the regions of space cannot, according to 

 Cheseaux, Olbers, and Struve, possess perfect and absolute 

 transparency. The results obtained by Sir William Herschel 

 from gauging the stars, 3 * and from his ingenious experi- 

 ments on the space-penetrating power of his great telescopes, 

 seem to show, that if the light of Sirius in its passage to us 



34 Traite de la Comete de 1743, avec une Addition sur la 

 force de la Lumiere et sa Propagation dans I' ether, et sur Lu 

 distance des etoiles fixes; par Loys de Cheseaux (1744). On 

 the transparency of the regions of space, see Olbers, in Bode's 

 Jahrbuch fur 1826, s. 110-121 ; and Struve, Etudes d'Astr 

 Stellaire, 1847, pp. 83-93, and note 95. Compare also 

 Sir John Herschel, Outlines of Astronomy, 798, and Cosmos, 

 vol. i. p. 142. 



83 Halley, On the Infinity of the Sphere of Fixed Stars, 

 in the Philos. Transact., vol. xxxi. for the ^ear 1720 

 pp. 22-26. 



54 Cosmos, vol. i. p. 70. 



