Einstein was aware of some of these exciting developments. He boldly entered the discussion, albeit in a novel way, when he added the last section to his groundbreaking June 1905 paper, “On the electrodynamics of moving bodies.”2 Without utilizing the aether or hypothesizing the physical characteristics of the electron, he employed his new theory, along with Maxwell’s equations, and F = ma (alas, an erroneous approach)to derive expressions for the transverse and longitudinal mass. Like most German-speaking scientists, he denoted mass by . We will simplify the notation and write the Lorentz factor as