20 Prof. ChalhVs Theory of the 



velocity, resolved perpendicularly to the line of pointing, to the 

 velocity of light. Now the communication above cited contains 

 an account of experiments made with telescopes the tubes of 

 which were partly filled with a transparent fluid, the object of 

 this arrangement being to ascertain what effect would thereby be 

 produced on the amount of aberration. 1 confess that I fully 

 expected, on account of the longer time occupied by the pass- 

 age of the light between the two points owing to the retarda- 

 tion caused by the fluid column, that the amount of aberration 

 would by this means be increased. But two independent sets of 

 experiments, one by M. Hoek with the telescope pointed to a 

 terrestrial* object, and the other by the Astronomer Royal with 

 the telescope directed to 7 Draconis, have proved conclusively 

 that the passage of the light through the fluid column does not 

 sensibly alter the amount of aberration. By parity of reasoning 

 the aberration is unaffected by transmission of the rays through 

 the lenses of the telescope. At the end of the aforesaid commu- 

 nication I have endeavoured to account for these facts theoreti- 

 cally, but not quite successfully. The following amended expla- 

 nation may, I think, be considered to be satisfactory. 



The rate of propagation of any disturbance of the aether being 

 supposed to be a in free aether, that within a medium whose re- 

 fractive index is a is -. Hence the mean retardation of the rate 

 of propagation by the reaction of the atoms is a 1 1 ), which, 



if a 2 represent the elasticity of the aether, may be supposed to be 



due to a retarding force represented by a 2 1 1 1 . Conceive 



the propagation to take place along the line of pointing of a tele- 

 scope in the tube of which there is a fluid column of refractive 

 index /*, and let the telescope be carried by the earth's motion 

 with a velocity the projection of which transverse to the line of 

 pointing is V. Hence the aether will be continually impelled by 

 the atoms of the fluid in the transverse direction proportionallv 

 to that resolved part of the earth's velocity. Like impulses will 

 be produced by the part resolved along the line of pointing ; but 

 as these, as far as regards aberration, would only give rise to 

 small quantities of the second order, they may be left out of 

 account. 



It has been already argued above that the disturbance of the 

 aether produced by the motion of translation of the earth is 

 steady motion of such kind that it is always quam proxime the 

 same at the same position relative to the earth's centre. Hence 

 the disturbance is propagated in space at the rate of the earth's 



