IOC COSMOS, 



jecture in a work whose intellectual author was far behind 

 his contemporaries in mathematical, astronomical, and phy- 

 sical knowledge. The velocity of reflected solar light was 

 first measured by Romer, (November, 1675,) by comparing the 

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

 of the direct light of the fixed stars was ascertained (in the 

 autumn of 1727) by means of Bradley's great discovery 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. 48 To these astronomical methods may be added one 



sound and light, Bacon says : " This last instance, and others 

 of a like nature, have sometimes excited in us a most marvel- 

 lous 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 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 ap- 

 peared so incredible to us that the images or radiations of 

 heavenly bodies could suddenly be conveyed through such 

 immense spaces to the sight, and it seemed that they ought 

 rather to be transmitted in a definite time. That doubt, how- 

 ever, as far as regards any great difference 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 Organum), p. 177. He then recals the 

 correct view he had previously announced precisely in the 

 manner of the ancients. Compare Mrs. Somerville's Connexion 

 of the Physical Sciences, p. 36; and Cosmos, vol. i. p. 145. 



45 See Arago 1 s explanation of his method in the Annuuire 

 du Bureau des Longitudes pour 1842, pp. 337343. " L'ob- 

 servation attentive des 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 minimum le change- 

 ment d'intensite s'opere lentement ; il est au contraire rapidc 

 a certaines epoques intermediares entre celles qui correspjn- 



