Mr. G. G. Stokes on the Aberration of Light. 9 



light and the direction of the motion of the aether at the point 

 considered is taken for the plane x z. Now, in consequence 

 of the motion of the aether, the direction of the light in space 

 will deviate from the normal to the wave through the angle 



-A in the contrary direction, as may be very easily shown 



(see Phil. Mag. for February 1846, p. 78). Hence the di- 

 rection of the light coming from a star is the same as that 

 of a right line drawn from the star, not merely at such a di- 

 stance from the earth that the motion of the aether is there 

 insensible, and again close to the surface of the earth, where 

 the aether may be supposed to move with the earth, but 

 throughout the whole course of the light; so that a ray of light 

 will proceed in a straight line even when the aether is in mo- 

 tion, provided the motion be such as to render (a.) an exact 

 differential. The orthogonal tmjectory represented in fig. 2 

 of my paper of July, must not be confounded with the path 

 of a ray of light. In that paper I supposed that the aether 

 close to the surface of the earth was at rest relatively to the 

 earth ; in fact, the very object of the paper was to get rid of 

 the apparent necessity of supposing the aether to pass through 

 the whole atmosphere and through the earth itself. It should 

 be observed, however, that the phaenomenon of aberration 

 allows us to suppose that the aether passes through the atmo- 

 sphere and through the earth with any velocity, either con- 

 stant, or varying from point to point, provided only (a.) be an 

 exact differential. . . 



P.S. I take this opportunity of adding a Pew words on the 

 subject of Prof. Challis's last communication. There is no- 

 thing so far as I can make out in which we differ, except the 

 sense in which we use the expression explaining a phaenome- 

 non from certain causes. According to my use of the term, 

 a person would be said to explain a phaenomenon when he has 

 shown that certain causes being assumed, the phaenomenon 

 would necessarily follow. In this sense we explain the forma- 

 tion of images in common optics, assuming the properties of 

 rays. We are able to show what must be the form, &c. of 

 the image. In this sense Prof. Challis has not explained 

 aberration by assuming merely the motion of the earth and 

 the velocity of light, since, for aught that appears from his 

 reasoning, a star might be displaced through double the angle 

 through which it is observed to be displaced. It was for this 

 reason, that in order to allow that Prof. Challis had explained 

 aberration, I attached, in a former communication, a peculiar 

 meaning to the word aberration. 



