Hereafter there will not be the slightest difficulty in dispensing with the assumption made in [sect:25.] Sec. 25 that the medium in question extends very far in all directions.
For after thermodynamic equilibrium has been everywhere established in our medium, the equilibrium is, according to the results of the last paragraph, in no way disturbed, if we assume any number of rigid surfaces impermeable to heat and rough or smooth to be inserted in the medium.
By means of these the whole system is divided into an arbitrary number of perfectly closed separate systems, each of which may be chosen as small as the general restrictions stated in [sect:2.] Sec. 2 permit.
It follows from this that the value of the specific intensity of radiation given in [eqn:(27)] (27) remains valid for the thermodynamic equilibrium of a substance enclosed in a space as small as we please and of any shape whatever.