22 Journal op^ the Mitchell Society [September 



through the interstices in and between molecules in the solid earth at 

 a rate sometimes of more than thirty miles per second without showing 

 the least sign of friction or binding of any kind, and so further experi- 

 ments were made. A\Yy tried the aberration experiment with a tele- 

 scope tube filled with water, and expected to see an increased aberra- 

 tion, because the velocity of light in water is only three-fourths of 

 what it is in air, and according to the aberration formula the less 

 the velocity of light in the telescope tube the greater the angle of 

 aberration should be. 



But when Airy measured the angle through which the water-slowed 

 light was deflected, he found it exactly the same as Bradley had foiuid 

 for full-speed light! This was a puzzling thing, to be explained only 

 on the theory that the water dragged the ether along with it, with a 

 velocity sufficient to compensate exactly for the change expected in the 

 angle from the diminished velocity of light. Time is wanting to detail 

 the experiments of Fizeau, Fresnel, and others to settle this point, but 

 they showed apparently that something of the kind does take place, 

 — a sort of entrainment or cling or viscous drag of the ether,^ — and 

 they strengthened enormously the conviction that the ether does exist, 

 is a substantial medium, and can be affected by matter, — that is, 

 dragged along. 



This brings ils to the celebrated experiment of Michelson and ]\Ior- 

 ley, which was an attempt made in 1881, and subsequently repeated, 

 to measure the absolute speed of the earth with respect to the station- 

 ary ether of space. They certainly had a right to expect success, for 

 they took every precaution to ensure it. The experiment consisted in 

 splitting a ray of light, sending one-half in the direction of the earth's 

 motion through space, then reflecting it by a mirror back to the source, 

 while the other half of the yslj is sent at right angles to the line of 

 motion of the earth, and is then similarly reflected back, the two 

 reflected rays interfering to produce interference bands in an inter- 

 ferometer, invented by Michelson. 



The distance from the source to each mirror is exactly the same, 

 but the distance the light travels to and fro between the mirrors and 

 the source is not the same when the earth is in motion, being greater 

 in the line-of-motion direction than in the direction at right angles to 

 it. The mirrors and other instruments were mounted on a heav.v stone 

 slab floated in mercury, and they expected that when the apparatus 

 was turned through a right angle so that what was before the longer 



