Primary glaucoma is a heterogeneous clinical entity, and the present paper deals with only one factor in its etiology—a possible mechanism of a vascular crisis in the eye, such as might result clinically in acute glaucoma. The most dramatic vascular crisis which can be induced experimentally is probably to produce a total cessation of the circulation and then to allow it to start again. This was brought about by raising the intra-ocular pressure to a height above that in the ophthalmic artery by the apparatus seen in figure 1. A cannula (A) leading into a glass chamber (B), the distal end of which was closed by a membrane (M) carrying a mirror, formed an optical manometer recording the variations in the intra-ocular pressure by a beam of light from a source (L) on a camera (C). The glass chamber was connected by tubing (D) to a four-way tube, the