100 The Lumini/erous Medium, 



this angle is to the sine of the visible inclination of the object 

 to the line in which the eye is moving, as the velocity of the eye 

 is to the velocity of light. Observations such as Bradley's will 

 therefore enable us to deduce the ratio of the mean orbital 

 velocity of the earth to the velocity of light, or, as it is called,. 

 the constant of 'aberration ; from its value Bradley calculated that 

 light is propagated from the sun to the earth in 8 minutes 

 12 seconds, which, as he remarked, "is as it were a Mean 

 betwixt what had at different times been determined from the 

 eclipses of Jupiter's satellites."* 



With the exception of Bradley's discovery, which was 

 primarily astronomical rather than optical, the eighteenth 

 century was decidedly barren, as regards both the experimental 

 and the theoretical investigation of light ; in curious contrast 

 to the brilliance of its record in respect of electrical researches. 

 But some attention must be given to a suggestive study f of the 

 aether, for which the younger John Bernoulli (b. 1710, d. 1790) 

 was in 1736 awarded the prize of the French Academy. His 

 ideas seem to have been originally suggested by an attempt}; 



*Struve in 1845 found for the constant of aberration the value 20"'445, which 

 lie afterwards corrected to 20"'463. This was superseded in 1883 by the value 

 20"-492, determined by M. Nyren. The observations of both Struve and Nyren 

 were made with the transit in the prime vertical. The method now generally 

 used depends on the measurement of differences of meridian zenith distances 

 (Talcott's method, as applied by F. Kiistner, Beobachtungs-Ergebnisse der kon. 

 Stern warte zu Berlin, Heft 3, 1888) ; the value at present favoured for the 

 constant of aberration is 20"-523. Cf. Chandler, Ast. Journal, xxiii, pp. 1, 12 

 (1903). 



The collective translatory motion of the solar system gives rise to aberrational: 

 terms in the apparent places of the fixed stars ; but the principal term of this 

 character does not vary with the time, and consequently is equivalent to a 

 permanent constant displacement. The second-order terms (i.e. those which 

 involve the ordinary constant of aberration multiplied by the sun's velocity) 

 might be measurable quantities in the case of stars near the Pole ; and the same is 

 true of the variations in the first-order terms (i.e. those which involve the sun's 

 velocity not multiplied by the constant of aberration) due to the circumstance 

 that the star's apparent R. A. and Declination, which occur in these terms, are 

 not constant, but are affected by Precession, Nutation, and Aberration. Cf. 

 Seeliger, Ast. Nach., cix., p. 273 (1884). 



t Printed in 1752, in the Recueil des pieces qui ont remportes les prix de V Acad.y. 

 tome iii. J Acta eniditorum, MDCCI, p. 19. 



