120.) A. A. Michelson—The relative motion of the Earth 
ArT. XXI.—The relutive motion of the Earth and the Luminif- 
erous ether ; by ALBERT A. MICHELSON, Master, U. 8. Navy. 
optical media, partakes of the motion of these media, to an ex- 
tent depending on their indices of refraction. For air, this 
motion would be but a small fraction of that of the air itself 
and will be neglected. 
ssuming then that the ether is at rest, the earth moving 
through it, the time required for light to pass from one point 
to another on the earth’s surface, would depend on the direc- 
tion in which it travels. 
Let V be the velocity of light. 
v = the speed of the earth with respect to the ether. 
D = the distance between the two points. 
d the distance through which the earth moves, while 
light travels from one point to the other. 
d, = the distance earth moves, while light passes in the 
opposite direction. 
iad te the direction of the line joining the two points to 
coincide with the direction of earth’s motion, and let T = time 
required for light to pass from the one point to the other, and 
Ho i 
T, = time required to perform the journey if the 
earth were at rest. 
Then Pe e. and T se aeamns pre d, 
x v Fhe v 
v 
From sear eee we find d=D-— and dD ry 
D v 
whence dd aaa aad 1 ¥a5 ; noed al, nearly, and 
_yizh 
pager. Sal 
If now it were possible to measure T'—T’, since V and T, are 
known, we could find v the velocity of the earth’s motion 
directions from earth; but that for this purpose the observa 
tions of these eclipses must greatly exceed in accuracy those 
