9. Deadness of the Optic Nerve to the Calorific Rays.
The layer of iodine used in the foregoing experiments, when placed before the eye, intercepted the light of the noonday sun. No trace of light from the electric lamp was visible, even in the darkest room, when a white screen was placed at the focus of the mirror. It was thought, however, that if the retina itself were brought into the focus the sensation of light might be experienced. The danger of this experiment was twofold. If the dark rays were absorbed in a high degree by the humours of the eye, the albumen of the humours might coagulate along the line of the rays. If, on the contrary, no such high absorption took place, the rays might reach the retina with a force sufficient to destroy it. To test the likelihood of these results, experiments were made on water and on a solution of alum, and they shewed it to be very improbable that in the brief time requisite for an experiment any serious damage could be done. The eye was therefore caused to approach the dark focus, no defence, in the first instance, being provided; but
