This is given in Eq. (2), which must be evaluated numerically. However, it is easily shown by hand in the high temperature or low frequency limit (5) that the obsolete derivation is completely wrong. Therefore the law could not have been tested experimentally with great precision. Incorrect mathematics cannot be tested experimentally with great precision. The error made by Lord Rayleigh and Sir James Jeans was to assume that the higher order infinitesimals (d omega) squared and (d omega) cubed can be neglected in the calculation of (omega + d omega)) cubed – omega cubed. As shown in this note they cannot be neglected at all because one is double integrated and the second is triple integrated.