The suitability of boric oxide glass containing phosphorescent materials for use as scintillation counters for slow neutrons is investigated both theoretically and experimentally. A theory is developed which can be used for predicting counting efficiencies of the phosphor disks given optical data and neutron absorption properties of the materials present, and physical details of the experimental arrangement. This is compared to experiment in one case with good agreement. It is shown that there are serious practical difficulties in the use of a boric glass phosphor as a slow neutron detector.