and
of our second system. But, just as in the first case, we should have been able to measure the velocity of light between the two points
and
of our second system. The principle of the invariant velocity of light states that in whatever Galilean system we might have operated, the measured velocity of light in vacuo would always be the same.
Of course this velocity may be positive or negative, according to whether the light ray is directed to the right or to the left; but we can obviate this ambiguity of sign by considering squared values.
The mathematical translation of this principle of physics yields us the following equation, which must remain invariably zero in value for all Galilean frames: