ETHER. 769 



might, by comparing the observed velocities in opposite directions, determine 

 the velocity of the aether with respect to these terrestrial stations. All 

 methods, however, by which ,it is practicable to determine the velocity of light 

 from terrestrial experiments depend on the measurement of the time required 

 for the double journey from one station to the other and back again, and the 

 increase of this time on account of a relative velocity of the aether equal to 

 that of the earth in its orbit would be only about one hundred millionth 

 part of the whole time of transmission, and would therefore be quite insensible. 



The theory of the motion of the aether is hardly sufficiently developed to 

 enable us to form a strict mathematical theory of the aberration of light, 

 taking into account the motion of the aether. Professor Stokes, however, has 

 shewn that, on a very probable hypothesis with respect to the motion of the 

 aether, the amount of aberration would not be sensibly affected by that motion. 



The only practicable method of determining directly the relative velocity of 

 the aether with respect to the solar system is to compare the values of the 

 velocity of light deduced from the observation of the eclipses of Jupiter's satellites 

 when Jupiter is seen from the earth at nearly opposite points of the ecliptic. 



Arago proposed to compare the deviation produced in the light of a star 

 after passing through an achromatic prism when the direction of the ray within 

 the prism formed different angles with the direction of motion of the earth 

 in its orbit. If the aether were moving swiftly through the prism, the deviation 

 might be expected to be different when the direction of the light was the 

 same as that of the aether, and when these directions were opposite. 



The present writer* arranged the experiment in a more practicable manner 

 by using an ordinary spectroscope, in which a plane mirror was substituted 

 for the slit of the collimator. The cross wires of the observing telescope were 

 illuminated. The light from any point of the wire passed through the object- 

 glass and then through the prisms as a parallel pencil till it fell on the 

 object-glass of the collimator, and came to a focus at the mirror, where it was 

 reflected, and after passing again through the object-glass it formed a pencil 

 passing through each of the prisms parallel to its original direction, so that 

 the object-glass of the observing telescope brought it to a focus coinciding 

 with the point of the cross wires from which it originally proceeded. Since 



* Phil. Trans. CLVIII. (1868), p. 532. [Communicated by Prof. Maxwell to Dr Huggins and 

 included by him in his paper on the spectra of some of the stars and nebulae.] 



VOL. II. 9? 



