Having determined in a series of articles that the principle of constancy of speed of light in vacuum of the Special Relativity Theory is false and has to be put aside because it cannot account for the loss in one of the two media which are in uniform rectilinear motion with respect to each other, it has become necessary to use different speeds of light in vacuum for different Galilean reference systems. In this paper this is demonstrated again by obtaining of a relation that relates the speed of light in vacuum c, constitutive parameters of the medium, frequency and the speeds of two media in uniform rectilinear motion with respect to each other. This in turn necessitates the revision of the concept of the permittivity of vacuum for different Galilean reference systems. Therefore first we determine the speed of light in vacuum for one of the two moving media, and define the permittivity of vacuum for that medium using the speed of light in vacuum found for that medium. We assume the permeability of vacuum is an invariant quantity given as mu(0) = 4 pi x 10(-7) [H/m]. The same procedure is repeated also for the second medium. The two media which are in relative motion with respect to each other and which are taken up in this paper, are a simple medium with loss and a perfectly conducting medium filling a half space. Their interface is an infinite plane perpendicular to the direction of the uniform rectilinear motion of the second medium which is constituted of the perfectly conducting half space. For simplicity we have assumed the incident wave that impinges on the interface has a wave vector that makes an angle of theta = pi/2 with the direction of the velocity of the moving medium and hence we can make use of the time dilation formula to transform the frequencies between the Galilean reference systems. The permittivity of vacuum and speed of light in vacuum results obtained are particular to this electromagnetic system.