In Reif's book Fundamentals of Statistical and Thermal Physics, he labels two formulas as the Stefan-Boltzmann Law. They are both involve T^4 but the constant is different. In one, on page 376, the law is given as (pi2/15)*(kT)4/(c*hbar)3. The other, on page 388, is (pi2/60)*(kT)4/(c2*hbar3). The second formula is c/4 times the first. I have looked carefully through the text and I can't see why there should be a difference. What have I overlooked?