Source of Terrestrial Magnetism, 21 



motion ; and any mutual action between the aether and the atoms 

 is propagated at the same rate. Consequently the rate of pro- 

 pagation resolved transversely to the line of pointing is the velo- 

 city V. Now since, as we have seen, when the independent rate 

 of propagation is a, the mutual action between the aether and the 



fol- 



/ 1\ 2 



atoms of the fluid column is represented by a 2 ( 1 J, it 



lows, analogously, that when the independent rate of propagation 

 is V, the mutual action between the sether and the atoms of the 



same fluid is V 2 ( 1 ) . This amount of apparent elasticity 



corresponds to a rate of propagation equal to V ( 1 ) in the 



transverse direction. Combining this with the rate of propaga- 

 tion - in the line of pointing, the result is an aberration from 



^ . . . / 1\ 



that line the angular measure of which is the ratio of V( 1 ) 



a V V >/ 



to -, or — (fj,— 1). By comparing the direction of the earth's 



motion with the direction of the propagation of light towards 

 the earth, it will be seen that this aberration is from the quarter 

 towards which the earth is moving, the ray being, as it were, 

 dragged towards that quarter while it traverses the part of its 

 course from the optical centre of the telescope to the field of 

 view, which lies within the fluid column. Relatively, there- 

 fore, to the usual mode of estimating aberration, this amount 

 is negative. 



V 



The instrumental aberration, as is known, is — , apart from the 



effect of the passage of the light through any medium in the 

 telescope-tube. For a medium of refractive index fM this ratio 



Yll 

 is altered to — -, on account of the rate of propagation in the 



medium being to that in vacuum in the ratio of 1 to fi. As 

 the sign of this aberration is positive, the total aberration is 



Ya V V 



— — (fju— 1), or — , which is independent of fi. Since the 



a a a 



same reasoning applies to any portion of a given medium, and 

 therefore to the whole of each medium, it follows, conformably 

 with experience, that the amount of aberration is unaffected by 

 the transmission of the light through any number of transparent 

 media contained within the telescope-tube. 



I cannot forbear remarking in conclusion, since the same mu- 

 tual action between the atoms of a substance and the sether has 



32174 



