Fragments of science : a series of detached essays, addresses, and reviews / by John Tyndall.

  • John Tyndall
Date:
1876
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Licence: Public Domain Mark

Credit: Fragments of science : a series of detached essays, addresses, and reviews / by John Tyndall. Source: Wellcome Collection.

    100/648 (page 48)
    9. Deadness of the Optic Nerve to the Calorific Rays. The layer of iodine used in the foregoing experiments intercepted the rays of the noonday sun. No trace of light from the electric lamp was visible in the darkest room, even when a white screen was placed at the focus of the mirror employed to concentrate the light. 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 absorp- tion 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 showed 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 the heat, acting upon the parts surrounding the pupil, could not be borne. An aperture was therefore pierced in a plate of metal, and the eye, placed behind the aperture, was caused to approach the point of convergence of invisible rays. The focus was attained, iirst by the pupil and afterwards by the retina. Eemoving the eye, but permitting the plate of metal to remain, a sheet of platinum foil was placed in the position occupied by the retina a moment before. The platinum colourless. To render the bulb of a thermometer a perfect absorbent, the glass ought in the first instance to be green. Soon after the discovery of fluorescence the late Dr. William Allen Miller pointed to the lime-light as an iUustration of exalted refrangibility. Direct experiments have since entirely confirmed the view expressed at page 210 of his -work on ' Chemistry,' published in 1866.