Corresponding to this electro-receptive sense, they had also cultivated the magnetic force of the eye. They had long known and investigated the exact relationship of light and electricity, and they could at any moment and place transform the one into the other. They had also observed ages before that even the commonest and weakest human eye had a faint luminosity in absolute darkness, and that any exertion of the will or passing wave of passion greatly increased it. Beside this fact they put the open secret that men of strong will and character differed from their fellows in the power of the eye, not only over human beings but over animals, and also the fact that the long-known plaything, mesmerism, had the eye as its chief organ. They came to the conclusion that the will was on its physical side a magnetic force, and that though most of its play was through the sense of touch, the muscular energies, and the voice, the eye was its highest and best channel. This inference was strengthened by noticing that amongst animals the fiercest-willed and most predatory could paralyse their victims by the exercise of some optic power, and as they prowled through the night, they had a perceptible glitter in their eyes that shone in the dark like lamps. They applied themselves to a minute and systematic investigation of the subject, and soon had instruments which would respond to the faintest ocular exercise of the will. They could measure any increase in the magnetic power of the eye; and before long it was observed that the subjects they experimented on grew rapidly in optic magnetism as they practised, and came to have a perceptible sheen in their eyes when they stood in the darkness. These men and women were found to have rapidly increasing power of sending anyone to sleep by gazing at him. At last all doubt vanished as to the new latent faculty which lay in the eye.