SU COSMOS. 



question whether those stars yet exist which we noAV see 

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

 jecture in a work whose intellectual author v/as far behind 

 his cotemporaries in mathematical, astronomical, and phys- 

 ical knowledge. The velocity of reflected solar light was 

 first measured by K-omer (November, 1G75) by comparing 

 the periods of occult ation of Jupiter's satellites ; while the 

 velocity of the direct light of the fixed stars Avas 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. t 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 U3 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 little after, and whedier there is not 

 (wdth 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 ditTerence 

 between the true and apparent time, was subsequently completely set 

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

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

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

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

 ences, p. 36, and Cosmos, vol. i., p. 154, 15.5. 



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

 des Longitndes pour 1842, p. 337-343. •' L'observation attentive des 

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

 la Vitesse de la lumiere de cette etoile. Pros du maximum et du mini- 

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

 rapide k certaines epoques intermediares entre celles qui correspondent 

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

 mentant d'eclat, passe pour la troisieme gi-andeur." 



" 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 m.aximum and the minimum the change of intensity is very 

 slow; it is, on the contrary, rapid at cei'tain intermediate epochs be- 

 tv/een those corresponding to the two extremes, when Algol, either di- 

 minifihing or increasing in brightness, appears of the third magnitude. 



