-axis, and having a constant value
for the acceleration with which all bodies in the field fall (i.e. characterized by conditions which approximately exist on the surface of the earth). According to Einstein's theory, any event will take place in this field in just the same way as it appears to occur when referred to a co-ordinate system which has an acceleration
in the positive direction of the
-axis. Now if a ray of light, the time of oscillation of which is
travels from a point