  Planck formula is considered as a first moment (average value) of unknown
function of electromagnetic energy distribution of black body radiation.
In-verse problem for the definition of the unknown function is solved for Gibbs
ensemble. The solution needs of ensembles with both absolute temperatures:
positive temperature and negative temperature. Such ensembles are the part of
more extended class of ensembles with finite energies and finite phase
vol-umes. In addition, the absence of Bohr - van Leeuwen paradox is considered
for such statistical ensembles.
