54 Prof. Cliallis on the Cause of the Aberration of Light. 



axes of all rays pass, and the point where the course of the ray 

 intersects the wire of the telescope. It would answer the pur- 

 pose equally well, and would perhaps be more distinct, to select 

 with the latter point the optical centre of the object-glass, 

 through which the axes of all rays incident upon the object-glass 

 from a star necessarily pass. 



Let be the position in space of the optical centre of the 

 object-glass at the instant when light from a star passes through 

 it, and let W be the position in space of the point of the wire on 

 which the same portion of light impinges. As it is a known 

 fact that light near the earth's surface travels through small 

 spaces sensibly in straight lines, the straight line joining and 

 W is the course of the ray in space. As it is also a known fact 

 that light occupies time in passing from one point of space to 

 another, the optical centre of the object-glass is carried by the 

 earth's motion to some position "&, during the transit of the 

 light from O to W. Now the instrument to which the telescope 

 is attached necessarily determines the direction of the ray to be 

 the straight line joining the two points ot and W, through which 

 it is known that the ray has passed. For these are points of the 

 instrument ; and that which the instrument performs is, to de- 

 termine the direction of the line joining these points with refer- 

 ence to certain fixed directions. But the course of the ray in 

 space is from to W. The angle OWot is aberration. The 

 constant of aberration is the ratio of Oot to OW, that is, the 

 ratio of the earth's velocity to the velocity of light. This ratio 

 is known by the theory of the earth's motion about the sun, and 

 by observations of eclipses of Jupiter's satellites. Thus the angle 

 OWot for a given star at a given time may be obtained by calcu- 

 lation and expressed numerically. The same angle may be 

 measured instrumentally. The two values when compared are 

 found to agree as nearly as possible, and thus aberration is 

 accounted for in as complete and satisfactory a manner as can 

 be desired. The explanation is a strict deduction from admitted 

 facts ; and the cause assigned for aberration, being a vera causa, 

 admits of no dispute. 



At the same time that I maintain the above to be the expla- 

 nation of aberration, I am fully sensible of the value of an expe- 

 riment, such as that of M. Fizeau, which appears to demonstrate 

 that the motion of bodies alters the velocity with which light 

 propagates itself in their interior. This fact must necessarily be 

 of great importance with regard to the modulatory theory of light, 

 but appears to me to be in no way inconsistent with the prece- 

 ding account of aberration. 



I am, Gentlemen, 



Cambridge Observatory, Your obedient Servant, 



• Dec 22, 1851. " J. Challis. 



