so COSMOS. 



question whether those stars yet exist which we now see 

 shining.* We are astonished to meet with this happy con- 

 jecture in a work whose intellectual author was far behind 

 his cotemporaries in mathematical, astronomical, and phys- 

 ical knowledge. The velocity of rejlected solar light wag 

 first measured by Romer (November, 1G75) by comparing 

 the periods of occultation of Jupiter's satellites ; while the 

 velocity of \\\q direct light of the fixed stars was ascertained 

 (in the autumn of 1727) by means of Bradley's great discov- 

 ery of aberration, which afforded objective evidence of the 

 translatory movement of the earth, and of the truth of the 

 Copernican system. In recent times, a third method of 

 measurement has been suggested by Arago, which is based 

 on the phenomena of light observed in a variable star, as, 

 for instance, Algol in Perseus. f To these astronomical meth- 

 ods may be added one of terrestrial measurement, lately con- 

 ducted with much ingenuity and success by M. Fizeau in 

 the neighborhood of Paris. It reminds us of Galileo's early 



* In speaking of the deceptions occasioned by the velocity of sound 

 and light, Bacon says : " This last instance, and others of a like nature, 

 have sometimes excited in us a most marvelous doubt, no less than 

 whether the image of the sky and stars is perceived as at the actual 

 moment of its existence, or rather a httle after, and whether there is not 

 (with regard to the visible appearance of the heavenly bodies) a true 

 and apparent place which is observed by astronomers in parallaxes. It 

 appeared so incredible to us that the images or radiations of heavenly 

 bodies could suddenly be conveyed through such immense spaces to the 

 eight, and it seemed that they ought rather to be transmitted in a def- 

 inite time. That doubt, however, as far as regards any great difference 

 between the tiiie and apparent time, was subsequently completely set 

 at rest when we considered . . . ." — The works of Francis Bacon, vol. 

 xiv., Lond., 1831 {Novum Organum), p. 177. He then recalls the cor- 

 rect view he had previously announced precisely in the manner of the 

 ancients. Compare Mrs. Somerville's Connection of the Physical Sci- 

 ences, p. 36, and Cosmos, vol. i., p. 154, 155. 



t See Arago's explanation of his method in the Annuaire du Bureau 

 des Longit^ides pour 1842, p. 337-343. " L'observation attentive dea 

 phases d'Algol a six mois d'intervalle servira a determiner directement 

 la vitesse de la lumiere de cette etoile. Pres du maximum et du mini- 

 mum le changement d'intensite s'opere lentement ; il est au contraire 

 rapide a certaiues epoques intermediares entre celles qui correspondent 

 aux deux etats extremes, quand Algol, soit en diminuant, soit en aug- 

 mentant d'eclat, passe pour la troisieme grandeur." 



' The attentive observation of the phases of Algol at a six-months in* 

 terval will serve to determine directly the velocity of that star's light 

 Near the maximum and the minimum the change of intensity is very 

 Blow; it is, on the contraiy, rapid at certain intermediate epochs b© 

 tween those corresponding to the two extremes, when Algol, either d' 

 minishing or increasing in bnghtuess, appears of the third magnitude 



