|What sustains the idea that the speed of light is invariant, is the
|Michelson-Morley interferometry experiment. The null result there
|apparently shows that the speed of the light in the direction of
|earth's motion is the same as the speed of light in the direction at
|right angles to earth's motion. Going by the conventional thinking
of
|the time, the speed of light should be c+v when emitted in the
|direction of earth's motion, and c-v when emitted in the direction
|opposite to earth's motion. Where c is the speed of light, and v is
|velocity of the earth.

There is a sign error here. Taking v to be the speed of the Earth
with
respect to the ether, the speed should be c-v in the direction of the
Earth's
motion with respect to the ether, and c+v opposite this direction.

Not at all. Just as a ball thrown at speed v from a platform moving
with velocity V moves at speed v+V with respect to a common frame, the
speed of light in the earth's direction has to be c+v. Why on earth
should it be c-v? Ether is a complete non-issue. It is by definition
simply a medium for the transmission of em waves.