36 TELOCITY OF LIGHT. [SECT. iv. 



some time to travel, we see Jupiter in conjunction, by means 

 of rays that left him 16 m 26 8 '6 before ; but, during that time, 

 we have changed or position, in consequence of the motion 

 of the earth in its orbit : we therefore refer Jupiter to a place 

 in which he is not. His true position is in the diagonal 

 (N. 97) of the parallelogram, whose sides are in the ratio of 

 the velocity of light to the velocity of the earth in its orbit, 

 which is as 190,000 to 19, or 10,000 to 1. In consequence 

 of the aberration of light, the heavenly bodies seem to be in 

 places in which they are not. In fact, if the earth were at 

 rest, rays from a star would pass along the axis of a telescope 

 directed to it ; but, if the earth were to begin to move in its 

 orbit with its usual velocity, these rays would strike against 

 the side of the tube ; it would, therefore, be necessary to in- 

 cline 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 aberra- 

 tion of the fixed stars perfectly corresponds with that given 

 by the eclipses of the first satellite. The same result, ob- 

 tained from sources so different, leaves not a doubt of its 

 truth. Many such beautiful coincidences, derived from cir- 

 cumstances apparently the most unpromising and dissimilar, 

 occur in physical astronomy, and prove connections which 

 we might otherwise be unable 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 con- 

 sists in the vibrations of an elastic fluid or ether filling space, 

 an hypothesis 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 fortu- 

 nate conjectures which have been confirmed by subsequent 



