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1. In this experiments a special apparatus to compress the eyeball was devised, and effects of pressure on the electrical excitability of the dark-adapted eye were investigated. 2. The intensity of the electrical test stimulus, a rectangular pulse of 0.1 second was increased step by step from a subthreshold level, and threshold voltages necessary to evoke the minimal electrical phosphene were determined. The electrical excitability or reciprocal of threshold was expressed in percentage of the resting level. 3. When pressure of 100-400g. was applied to the eyeball, the electrical excitability decreased rapidly within a few seconds, and then remained constant for about 4-5 seconds, to be followed by a second rapid decrease. The steady level mentioned above remained always above 50% of the resting level. There was a linear relation between the intensity of pressure and the decrease of excitability caused by the pressure. 4. The effect of pressure of 1-10 seconds in duration and 100-200g. in intensity remained unchanged about 3 seconds after removal of the pressure, and it took 5-10 seconds for complete recovery. 5. With the same pressure apparatus the critical value of pressure required to stop retinal circulation was determined by means of an ophthalmoscope in three subjects, and values 460-480 g. were obtained. 6. In order to investigate the effect of pressure on the photosensitivity, dark-adaptation curves were determined under pressure by Nagel's adaptometer, and it was found that light thresholds were increased by pressure in both cone- and rod-branches of dark-adaptation curves. All these findings have provided evidence that the electrical excitability of the eye is determined by the state of the retina. The writer is greatly indebted to Prof. K. Motokawa, under whose guidance this investigation was performed, for his generous advice and valuable suggestions throughout the course of the experiment and the preparation of the manuscript.

Effects of Mechanical Pressure upon Electrical Excitability of Human Eye