The coincidence rate of a perfectly efficient two‐counter telescope embedded in an isotropic field of radiation is obtained in terms of the radiation intensity and the effective dimensions and separation of the counters comprising the telescope. By means of the formulas the counting rate may be located between an upper and a lower bound which in most practical cases differ by only a few percent. The information embodied in the formulas is also presented graphically in a form convenient for ready calculation of the ratio of coincidence rate to radiation intensity once the effective dimensions and separation of the two counters are known.