suddenly increased and then fell to a point lower than it had hitherto been. We repeat the experiment and you observe we get the same results. A sudden influx of light causes an increase in the current of the living eye; under the continued action of light the retina becomes fatigued, and, when light is taken off, there is another increase and then a great falling off. You see how sensitive the eye is—even a flash or striking a match near it produces the effect. If I let the light pass through this bit of red glass we get an effect almost equal to what we got with yellow light from the taper; but if it passes through this dark blue glass, you notice the effect is much less. This is perhaps the most delicate experiment in the range of physiological science.
We have now examined the muscle-current, the nerve-current, the heart-current, and the eye-current. Let me next endeavour to show you the man- current. I have here two flat vulcanite troughs into which we have poured a three-quarters per cent solution of common salt. Mr. Brodie has placed one zinc trough by the side of each flat trough, and he has dipped the points of the clay pads into the salt
