An essential feature is that the constancy of the velocity of light is not demanded in either of the postulates 1 and 2. Rather, the distinguishing property of a definite velocity in virtue of which it preserves its value in all systems that emerge out of one another through such transformations is a direct corollary to these two general postulates, and the result of the Michelson-Morley experiment merely determines the value of this special velocity which could, of course, be found only from observation.
[Note 5] (p. 15). Einstein has shown in a simple example how, on the basis of the formulæ of the special theory of relativity, a point-mass loses inertial mass when it radiates out energy.
We assume that a point-mass emits a light-wave of energy
in a certain direction, and a light-wave of the same energy
in the opposite direction. Then, in view of the symmetry of the process of emission with respect to the system of reference of the co-ordinates
,
