Prof. Challis on the Aberration of Light. 



431 



'Natural Philosophy' (p. 294), but is not accompanied by proof, 

 and I have not met with any proof of it elsewhere. In order to 

 prove the rule completely, it is necessary first to explain the 

 cause to which aberration is due. In vol. iii. (1852) of the 

 Philosophical Magazine (p. 53), I have proposed an explanation 

 of aberration which may be briefly stated as follows. In every 

 determination of the apparent place of a celestial object by an 

 optical instrument, the light from the object passes actually or 

 virtually through two points rigidly connected with the instru- 

 ment. One of these is necessarily the optical centre of the 

 object-glass, and the other is a point in the field of view of the 

 telescope, which we may suppose to be coincident with the inter- 

 section of two wires. Let, at the instant of an observation, O 

 be the position of the first point, and W that of the other. Then 

 the instrument gives by its graduation and by a note of time, 

 the direction of the line joining and W referred to certain 

 fixed directions. If the transmission of light were instantaneous, 

 this direction would coincide with the path of the light, sup- 

 posing the path to be rectilinear in the short space OW. But 

 if light occupies a sensible time in passing between the two 

 points, in that time the position of the optical centre of the 

 object-glass has changed, by reason of the earth's motion, from 

 0' to ; so that and W being simultaneous positions of the 

 direction-points, the instrument gives OW for the path of the 

 light, the real path being O'W. It is found by observation that 

 the difference between these two directions is the whole amount 

 of the aberration of a star. This general explanation of aberra- 

 tion being admitted, the following is the proof of the rule above 

 mentioned. 



Let e and p be simultaneous 

 positions of the observer and the 

 object, and while light is tra- 

 velling from p to the observer, 

 let the observer be carried by 

 the earth's motion from e to E. 

 Then, leaving out of considera- 

 tion any causes, such as atmo- 

 spheric refraction, which may 

 affect the course of the ray 

 before it enters the telescope, 

 the object is seen by the ob- 

 server when at E by a r ly which 

 has described t lie path ;;E. Draw 

 pQ parallel ;md equal to <?E, and 

 join EQ. Thus by reason of 

 aberration as above explained, the instrumental direction of the 



