1892.] 



on the Motion of the Ether near the Earth. 



679 



the whole length of 30 feet ; while the other half beam, viz. that 

 travelling against the motion of the disks, is retarded, or its path 

 virtually lengthened, by the same amount. 



If this acceleration and retardation actually occurs, waves which 

 did not interfere on meeting, before the disks moved, will interfere now, 

 for one will arrive at the common goal half a length behind the other. 



Now a gradual change of bright space to dark, and vice versa, 

 shows itself, to an observer looking at the bands, as a gradual change 

 of position of the bright stripes, or a shift of the bands. A shift of 



Fig. 10. 



Plan of Steel Disks, one yard in diameter, and Optical Frame. 



the bands, and especially of the middle white band, which is much 

 more stable than the others, is what we look for. 



At first I saw plenty of shift. In -the first experiment the bands 

 sailed across the field as the disks got up speed until the crosswire 

 had traversed a band and a half. The conditions were such that had 

 the ether whirled at the full speed of the disks I should have seen a 

 shift of three bands. It looked very much as if the light was helped 

 along at half the speed of the moving matter, just as it is inside water. 



On stopping the disks the bands returned to their old position. 

 On starting them again in the opposite direction, the bands ought to 

 have shifted the other way too ; but they did not ; they went the 

 same way as before. 



The shift was therefore wholly spurious; it was caused by the 

 centrifugal force of the blast of air thrown off from the moving disks. 

 The mirrors and frame had to be protected from this. Many other 



