A new principle is derived from mathematical considerations which will allow the calculation of the number of bodies contained in the unit volume by counting the number of transections on the unit area of a random section. The simple equation involves two coefficients related to the volumetric density of the bodies in the volume, and to their configuration. Means for the accurate determination of these coefficients are given. The verification of this principle in model experiments showed a high degree of reliability of the method with average errors of estimate of 2–4%. The method has been applied to the counting of alveoli in five normal human lungs, which were found to number an average of 300 million, with a striking constancy from lung to lung.

Submitted on May 31, 1961