Prof. Powell on the Theory of the Aberration of Light. 439 



In the case of aberration, the only difference perhaps is that 

 the facts which explain the aberration are so much more simple 

 and familiar that we feel satisfied almost without any theory 

 of them at all. 



Or again: — the retardation owing to light entering a denser 

 medium, as the humours of the eye, and the question of 

 its influence on the aberration, were discussed at an early 

 period by Melvill (1753) and Wilson (1782) on the theory of 

 emission and attraction ; who for a long time did not perceive 

 the compensation effected by the greater refraction*: this is 

 in itself independent of theory. Yet the theoretical investiga- 

 tion was not unimportant. 



Boscovichf also had maintained that an object viewed 

 through a refracting medium in motion with the observer, will 

 change its apparent place; and both he and Prof. Wilson had 

 proposed to try it by means of a telescope filled with water. 

 But Mr. Robisonj;, besides pointing out the impracticability 

 of this project, showed on theoretical grounds an oversight in 

 the reasoning (though he had himself for a time embraced the 

 idea), and concluded by establishing, on the emission theory, 

 this proposition : — " If a ray of light moving in any direction, 

 and with any velocity, meet with the surface of a refracting 

 medium while it is in motion, its final relative motion will be 

 the same as if the medium had been at rest, and the light had 

 approached it with the same initial relative motion." (P. 106.) 

 And questions somewhat akin to these have occupied the at- 

 tention of more recent inquirers in connexion with the prin- 

 ciples of undulations; and especially the conclusion that the 

 laws of reflexion and refraction in general are uninfluenced by 

 the motion of the aether along with the earth, has formed the 

 subject of one portion of Mr. Stokes's investigations. And 

 that such investigations are requisite for a complete and satis- 

 factory theoretical view of the case, must on all hands be ad- 

 mitted. 



Or to take a different illustration ; — when the composition of 

 forces and the experimental law of falling bodies were known, 

 the explanation of the pendulum was complete, without any 

 reference to the theory of universal gravitation. But it was 

 incumbent on that theory to explain the law of falling bodies: 

 and when it did so, it explained the pendulum. No one how- 

 ever doubts that it is highly valuable and satisfactory thus to 

 connect the pendulum with universal gravitation. Here, in- 

 deed, the case is far stronger than in those of light, since we 



* See Rigaud's Memoir of Bradley, p. xxxiii. and 483. 



t Miscell. Works, vols. ii. and iv. 1785. % Ed. Trans, ii. 83, 1788. 



