SKCT. IV. VELOCITY OF LIGHT. 31 



would, therefore, be necessary to incline the telescope 

 a little, in order to see the star. The angle contained 

 between the axis of the telescope and a line drawn to 

 the true place of the star, is its aberration, which varies 

 in quantity and direction in different parts of the earth's 

 orbit ; but as it is only 20"-36, it is insensible in ordinary 

 cases (N. 98). 



The velocity of light deduced from the observed aber- 

 ration of the fixed stars perfectly corresponds with that 

 given by the eclipses of the first satellite. The same 

 result, obtained from sources so different, leaves not a 

 doubt of its truth. Many such beautiful coincidences, 

 derived from circumstances apparently the most un- 

 promising and dissimilar, occur in physical astronomy, 

 and prove connections which we might otherwise be un- 

 able to trace. The identity of the velocity of light, at 

 the distance of Jupiter, and on the earth's surface, shows 

 that its velocity is uniform ; and if light consists in the 

 vibrations of an elastic fluid or ether filling space, a hy- 

 pothesis which accords best with observed phenomena, 

 the uniformity of its velocity shows that the density 

 of the fluid throughout the whole extent of the solar 

 system must be proportional to its elasticity (N. 99). 

 Among the fortunate conjectures which have been con- 

 firmed by subsequent experience, that of Bacon is not 

 the least remarkable. " It produces in me," says the 

 restorer of true philosophy, " a doubt whether the face 

 of the serene and starry heavens be seen at the instant 

 it really exists, or not till some time later : and whether 

 there be not, with respect to the heavenly bodies, a true 

 time and an apparent time, no less than a true place 

 and an apparent place, as astronomers say, on account 

 of parallax. For it seems incredible that the species or 

 rays of the celestial bodies can pass through the im- 

 mense interval between them and us in an instant, or 

 that they do not even require some considerable portion 

 of time." 



Great discoveries generally lead to a variety of con- 

 clusions : the aberration of light affords a direct proof of 

 the motion of the earth in its orbit ; and its rotation is 

 proved by the theory of falling bodies, since the centri- 

 fugal force it induces retards the oscillations of the pen- 



