Light; a course of experimental optics, chiefly with the lantern . onstrate the rapid andminute motions produced by heat. Upon the table weadjust a Trevelyan rocker, a, with its block of lead, l, andfulcrum-knob as usual. Having heated it, we fix on itsface, by any cement that will bear the heat, a small mirrorB, like that just used, and by our glass reflector, c, directthe beam of light from a small aperture down upon it at anyangle, so as to be reflected to the ceiling or the screen, D 34 LIGHT. [chap. where the spot should be focused sharply. The wholearrangement is shown in Fig. 23. What w
Light; a course of experimental optics, chiefly with the lantern . onstrate the rapid andminute motions produced by heat. Upon the table weadjust a Trevelyan rocker, a, with its block of lead, l, andfulcrum-knob as usual. Having heated it, we fix on itsface, by any cement that will bear the heat, a small mirrorB, like that just used, and by our glass reflector, c, directthe beam of light from a small aperture down upon it at anyangle, so as to be reflected to the ceiling or the screen, D 34 LIGHT. [chap. where the spot should be focused sharply. The wholearrangement is shown in Fig. 23. What would be a sta-tionary spot of light if the rocker were cold, is by the smallrocking motion at once prolonged (by the persistence ofimpressions on the retina) into a bright line of light, andby gradually raising with the hand the block d bearing thefulcrum end, this is converted into a beautiful wavy lineE F, which makes every separate motion visible. Therocker should be judiciously chosen by experiment as toits period of vibration. A kitchen poker may be made to. Fig. do by bevelling one side of the square end to a very obtuseangle, and attaching the mirror to the opposite face. Our next illustrations may be from those vibrationswhich produce sound. The elegant experiment of may easily be illustrated with the lantern andsuch a tuning-fork as may be bought for 5.;. On the out-side of the end of one prong must be strongly cemented asmall bit of looking-glass, and on the other a bit of metalor glass to balance it. We then mount the fork in a II.] THE REFLECTING MIRROR. 35 heavy block of lead, place a slide with a small hole in theslide-stage, and arrange the whole with the plane reflector inthe vertical socket, as in Fig. 34, so that the light from thelantern may be reflected back from the mirror on the forkA to that on the pillar-stand b, and thence to the screen,where it produces a spot, which must be focused. Thecard screen should also be placed between the fork and
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