They have been observed however.

Suppose we compare two observations on the velocity of light made with the same apparatus as we turn it through a right angle.

The velocity of the earth relatively to the sun is in one direction, the velocity of light relatively to the ether should be the same in all directions.

Hence if space when we take the ether as at rest means the same thing as space when we take the earth as at rest, we ought to find that the velocity of light relatively to the earth varies according to the direction from which it comes.
These observations on earth constitute the basic principle of the famous experiments designed to detect the motion of the earth through the ether.

You all know that, quite unexpectedly, they gave a null result.
This is completely explained by the fact that, the space-system and the time-system which we are using are in certain minute ways different from the space and the time relatively to the sun or relatively to any other body with respect to which it is moving.
All this discussion as to the nature of time and space has lifted above our horizon a great difficulty which affects the formulation of all the ultimate laws of physics--for example, the laws of the electromagnetic field, and the law of gravitation.