The Davisson-Germer experiment is the classical experiment that verifies the formula of De Broglie: λ = h/mv It is proposed here that a feasible modification in the original Davisson-Germer experiment could finally decide If Relativity Theory is right or wrong. Detailed descriptions of the Davisson-Germer experiment can easily be found in the web The original experiment uses the constant (“rest”) mass of the electrons and the classical formula for the Kinetic Energy in the applied formula. The classic magnitudes give exact results. The electrons are accelerated by only 50 volts to reach velocities about 1% of the constant c. It must be considered that even at those velocities some mass variation should be detected but also it is easy to accelerate them more and precisely verify any dependency between the mass and the velocity of the electrons. I can reasonably speculate why this experiment haven't been performed for higher velocities by both Relativity defenders or fighters to verify the if the mass of the electrons varies with velocity as sustained by Relativity or not? This is very strange! Try to state that only the "rest mass" must be considering in the De Broglie formula is not aceptable. The original development of the formula determines that the "relativistic mass" must be used. I suggest to add a Velocity Selector in the Davisson-Germer apparatus just after the acceleration stage to measure the velocities directly. A Velocity Selector is a stage were the electrons pass through both an Electric Field that produces a force to deflect them in one direction and a Magnetic Field that produces a force to deflect the electrons in the opposite direction. At the end of the stage there's a hole where only the electrons of a precise velocities can pass through. The Magnetic Force is velocity dependent and so the Electric Force can be adjusted to compensate the effect of the Electric Force. It's another method to mesure the velocity. There's nothing to loose and a lot to win performing the new version of the experiment. At least more precise results will be determined. It's a very interesting idea to be consider by someones who have the resources to make a new apparatus and perform the suggested experiment. I can't.