It has been known for the last thirty-four years that when the fresh excised eyeball of a frog is connected by non-polarizable electrodes with a sensitive galvanometer, an electrical current (travelling through the eye from the retina to the cornea) may be detected, and that variations occur in this current, due solely to the action of light on the retina.
It is also well known that salts of radium are luminous in the dark, and that if a tube containing radium† is pressed against the closed lid of the eyeball, or even pressed against the temple, one has the consciousness of luminosity. This being so, it was of interest to ascertain whether this luminosity was due to the radium causing fluorescence of any of the structures of the eyeball, or whether it was due to the direct action of one or other of the emanations of radium on the retina itself.