THE PROPAGATION OF LIGHT. 35 



the high number yielded by the repetition of this unit. 

 Bessel 12 very truly observes that " the distance which light 

 traverses in a year is not more appreciable to us than the 

 distance which it traverses in ten years. Therefore every 

 endeavour must fail to convey to the mind any idea of a 

 magnitude exceeding those that are accessible on the earth." 

 This overpowering force of numbers is as clearly manifested 

 in the smallest organisms of animal life as in the milky way 

 of those self-luminous suns which we call fixed stars. What 

 masses of Polythalamiae are inclosed, according to Ehren- 

 berg, in one thin stratum of chalk ! This eminent investi- 

 gator of nature asserts that one cubic inch of the Bilin 

 polishing slate, w r hich constitutes a sort of mountain cap 

 forty feet in height, contains 41000 millions of the micro- 

 scopic Galionella distans; while the same volume contains 

 more than 1 billion 750000 millions of distinct individuals 

 of Galionella ferruginea. 13 Such estimates remind us of the 

 treatise named Arenarius tyap.^^?) of Archimedes of the 

 sand-grains which -might fill the universe of space ! If the 

 starry heavens, by incalculable numbers, magnitude, space, 

 duration, and length of periods, impress man with the con- 

 viction of his own insignificance, his physical weakness, 

 and the ephemeral nature of his existence; he is, on the 

 other hand, cheered and invigorated by the consciousness of 

 having been enabled, by the application and development of 

 intellect, to investigate very many important points in refer- 

 ence to the laws of Nature and the sidereal arrangement 

 of the universe. 



Although not only the propagation of light, but also a special 

 form of its diminished intensity, the resisting medium acting 



2 Bessel, in Schumacher's Jahrluchfur 1839, s. 50. 



3 Ehrenberg, Abhandl der Eerl. Akad., 1838, s. 59; also 

 in his Infusionsthiere, s. 170. 



D2 



