Lorentz applied his mathematical talents to the discovery of the necessary adjustments which would have to exist in nature for this correlation to be satisfied completely so far as electrodynamics and optics were concerned. In order to understand the significance of his investigations, we must recall what we have said of the laws of nature and of the space and time transformations. We remember that Newtonian relativity expressed the fact that mechanical phenomena, hence the laws of mechanics regulating these mechanical phenomena, remained unchanged when we passed from one Galilean frame to another. The significance of this statement is as follows: To begin with, when we wish to fix the position of a body in our frame, it is also necessary to specify the instant of time at which the body occupies this position. We thus have four co-ordinates to take into consideration: three for space, , , ; and one for time, . If, now, we change our frame of reference, taking our stand in some other Galilean system, which is in motion with respect to the first, these co-ordinates will change in value or will be transformed. For instance, if a body remains at rest at the meeting point of the three axes of our first frame, its co-ordinates will always be , , in this frame for all values of the time . But computed with respect to our second frame, the new co-ordinates represented by , , will vary with time, since with respect to this second frame the body is moving—i.e., is adopting successive positions in time. The classical Galilean transformations had for their object to tell us exactly how these co-ordinates would vary when we passed from one Galilean frame to another. As it was always assumed by classical science that time was absolute, the same for all systems, the only co-ordinates susceptible of variation as we changed frames were the space co-ordinates , , . As for , it had the same value in all frames, and this was expressed by writing . Now the laws or equations which govern phenomena contain in their expression, among other quantities, these co-ordinates, , , , . Hence, in the general case it is only to be expected that the laws will change in form when we refer them first to one Galilean frame, then to another; or again, when we replace , , , by , , , . When we say that the laws suffer no change in form or are transformed into themselves we mean that this substitution of the new co-ordinates for the old does not modify the form of the equations or laws.