Assuming the light as composed of longitudinal-extended elastic (and massive) particles emitted during an emission time T at speed c (=u) (with u the escape speed from all the masses, toward the infinity), it is shown that c is invariant (under the Newtonian mechanics laws), for an Observer fixed to the initial emission point Ep (the point where the emission starts), in spite of any motion of the source (of light) with respect to Ep. On the contrary, an Observer, in motion from Ep during the emission, will state (indirectly) a Galilean variation of c which can be proved and evaluated by an appropriate feasible experiment described here.