  We study physical situation considered by Einstein (Ann. Physik, 17, 1905)
for his first derivation of mass-energy equivalence. Einstein introduced a
constant $C$ in his derivation and reasoning surrounding $C$ and equations
containing $C$ caused criticism by Ives. Here we clarify Einstein's derivation
and obtain a value for constant $C$. The obtained zero value for $C$ suggests
alternative explanation for Einstein's derivation and makes Ives's criticism a
void and for which details are also presented in this paper.
