The theory that scattering in the refractive media of the eye is responsible for the observed increase in the brightness difference threshold when a point source of light of high intensity is introduced into the field is developed in detail, and the theoretical predictions compared with experimental determinations of the threshold. The theory gives a much less rapid falling off in the glare effect with increase of the glare angle (angular separation between glare source and fixation point) than is actually observed. The amount of light which must be lost by scattering in the eye media, in order to account for the absolute magnitude of the observed glare effect, comes out more than double the total loss from all causes (estimated from measure- ments of eye absorptions on bullocks'' eyes). It is concluded that scattering in the eye media can play only a minor role in raising the brightness difference thresholds.