A quantitative approach to measuring events in the retinal circulation is now possible. Caliber changes of retinal vessels ranging in size from 25μ to 110μ can be documented with considerable precision by measurements made from color photographs. The velocity of flow in the retinal arterioles can be measured with moderate accuracy by determining the velocity of the leading edge of the axial stream of fluorescent dye by frame analysis of cinephotographs obtained by using an image intensifier with a light gain of approximately half a million times. The velocity of flow in the human superior temporal arteriole is between 20 and 40 mm/sec with volume flow rate of approximately 7μl/min.