Rev. J. Challis on the Aberration of Light. 91 



pose equally well if I had fixed upon a division of the gradua- 

 tion of a circle used for astronomical observation. The fol- 

 lowing method of viewing the subject will put this in a clear 

 light:— 



Let apq be a circle of given radius, the centre of which 

 coincides with the place of the spectator's eye, and partakes of 

 the earth's motion. Let the plane of the circle pass through 

 e 1 c y, the line in which the eye moves, and through sp' e, the 

 line in which light comes to the eye from a star. To find the 

 point of the circle on which the light impinges by which the 

 star is seen, take e 1 e equal to the space passed over by the 

 eye while light travels over the radius of the circle; take e' p 1 



equal to the radius of the circle ; draw ep parallel to e 1 ' p', and 

 join p' p. Then p is the point required; for ftp is evidently 

 equal and parallel to e 1 e. Consequently p is the point of the 

 circle which is seen in coincidence with the star s. If the star 

 were at 5' in the direction of the line e' eq produced, the point 

 q on that line would be seen in coincidence with it. Now 

 suppose the circle to be graduated, and to be used by an 

 astronomer to measure the angle ses' ; then most certainly he 

 would read off' the angle peq instead of the angle pi e q, be- 

 cause to turn a given point of the circle from apparent coin- 

 cidence with one star to apparent coincidence with the other, 

 the circle would be moved through the angle p eq. The dif- 

 ference between the two angles, viz. sep, is equal to the 

 amount of aberration determined by astronomical observation. 

 The phenomenon is thus entirely accounted for. 



It follows as a corollary from this reasoning, that the di- 

 rection sp' e of the progression of light from a celestial object, 



H 2 



