Prof. Powell on the Theory of the Aberration of Light. 95 



instant of its course that ray has one point coinciding with a 

 point in the axis of the tube in motion. Thus by composi- 

 tion of the two motions, this ray, though by a different^ woc?z« 

 opera7idi, comes down the axis of the telescope : whether this 

 will, in theory, be the direction in which the eye sees both 

 the star and wire, will depend on the consideration of a simi- 

 lar question to that stated in p. 433. 



But there can be no doubt that in point of fact the axis of 

 the telescope is the direction to which the eye (by whatever 

 means) is led to refer the two objects seen together by rays 

 pursuing a different route, relatively to space. 



Prof. Challis has favoured me with the following additional 

 illustration. 



"Suppose two ships to be sailing due north at the same 

 rate, and one to be due east of the other : a boat is despatched 

 from the eastern ship to the western, and goes in a rectilinear 

 course from the one to the other. If the crew of the western 

 ship, seeing the other ship due east, and knowing that the 

 boat came from her, concluded that the course of the boat had 

 been exactly from east towards west, they would plainly be 

 mistaken. It is precisely this error that the astronomer com- 

 mits." 



Among some valuable remarks on the entire subject which 

 I have received from Mr. Stokes, I learn that the very cursory 

 statement I made as to the difference between the theoretical 

 investigations of that gentleman and Prof. Challis, does not 

 convey a perfectly accurate idea of the point at issue ; I am 

 therefore anxious to represent it more correctl}' as follows : — 



Mr. Stokes at first contended that it was necessary to sup- 

 pose the expression {u d x + Sic.) to be an exact differential, 

 while Prof. Challis rejected that restriction. Subsequently 

 however Prof. Challis agreed in Mr. Stokes's view: but he 

 showed that, on that supposition, the expression for the change 

 in the direction of the normal is such as to allow us to sup- 

 pose the motion of the jfither at the surface of the earth rela- 

 tively to the earth to be of any amount. But Mr. Stokes 

 differs in opinion as to \.\\e fact ; conceiving it most probable 

 that the aether within the earth and close to its surface is at 

 rest relatively to it. 



In my paper however I professedly avoided entering on 

 this part of the question ; and the remark just made in no 

 way affects the other parts of my observations. 



