Sound / by John Tyndall.

  • Tyndall, John, 1820-1893.
Date:
1883
Catalogue details

Licence: Public Domain Mark

Credit: Sound / by John Tyndall. Source: Wellcome Collection.

Provider: This material has been provided by UCL Library Services. The original may be consulted at UCL (University College London)

    37/494 (page 15)
    REFLECTION OF SOUND BY MIRRORS. that in optics the positions of a body and of its image are reversible. When a candle is placed at this lower focus, you see its image on the gallery above, and I have only to turn the mirror on its stand to make the image of the flame fall upon any one of the row of persons who occupy the front seat in the gallery. Removing the candle, and putting the watch, w, fig. 8, in its place, the person on whom the light fell distinctly hears the sound. The ' conjugate mirrors,' employed to illustrate the re- flection of light and radiant heat, are also applicable here. F13. 8. One of these two parabolic mirrors, n ri, fig. 9, is placed upon the table, the other, m m', being drawn up to a height of five-and-twenty feet. When the electric light U placed in the focus a of the lower mirror, a parallel beam rises like a luminous pillar to the upper mirror, which brings the beam to a focus. At that focus is seen a spot of sunlike brilliancy, due to the reflection of the light from the surface of a watch, w. The watch is ticking, but I do not hear it. At this lower focus, a, however, we have the energy of every sonorous wave converged. Placing at a (he ear, or better still a funnel with a tube reaching to the