Given the known shape of the radiation field given by Planck's law, Einstein could calculate the mean square energy fluctuation of the black-body radiation. He found the root mean square energy fluctuation ⟨ ϵ ϵ 2 ⟩ in a small volume v of a cavity filled with thermal radiation in the frequency interval between ν ν and ν ν + d ν ν to be a function of frequency and temperature: