Consider a light source fixed in space XYZ. An observer facing plane y in z direction can only see photons from y plane moving in the direction x and y. Other planes are not visible and only the light speed of y plane in the direction of x or y can be measured at right angle to y and x. This of course will be constant (c1) for the medium used. If the observer or the light source moved at any speed towards or away from each other, no relative speed is detected due to the fact that the movement is perpendicular to y plane and the photons in y plane moving in the direction x and y.