A knowledge of ocular blood flow is of fundamental importance for a complete understanding of the function of the eye in both health and disease. While this fact is well appreciated, investigation of ocular blood flow has been extremely limited. The main reason for this lack of research is no doubt due to the relative inaccessibility and small volume of the vasculature of the eyeball, thus rendering impractical for ocular application most of the usual methods of blood-flow analysis. Perkins1 and Von Sallmann,2 using a small thermocouple, measured changes in ciliary blood flow by the determination of temperature changes in that region. Alterations in the electroretinogram3,4 and flicker fusion frequency5 have also been utilized as indicators of blood flow changes. Recently, Sapirstein6 has developed a new technique for the determination of blood flow that seems suitable for a study of ocular blood flow. Sapirstein states