Notices of the proceedings at the meetings of the members of the Royal Institution of Great Britain with abstracts of the discourses . itude on which theperfection of the shadow depends; the size of the obstacle, and the Vol. XVII. (No. 98.) 2 g 428 Lord Bayleigh [Jan. 15, distance across which the shadow is thrown, must also be taken intoconsideration. The optical shadow of a small object, thrown acrossa considerable distance, partakes of the imperfections generally ob-served in connection with sound shadows. It was calculated by the French mathematician, Poisson, that,according to the wave t
Notices of the proceedings at the meetings of the members of the Royal Institution of Great Britain with abstracts of the discourses . itude on which theperfection of the shadow depends; the size of the obstacle, and the Vol. XVII. (No. 98.) 2 g 428 Lord Bayleigh [Jan. 15, distance across which the shadow is thrown, must also be taken intoconsideration. The optical shadow of a small object, thrown acrossa considerable distance, partakes of the imperfections generally ob-served in connection with sound shadows. It was calculated by the French mathematician, Poisson, that,according to the wave theory of light, there should be a bright spotin the middle of the shadow of a small circular disc—a result thatwas thought to disprove the wave theory by a reductio ad unknown to Poisson, this very phenomenon had actuallybeen observed some years earlier, and was easily verified when asuitably arranged experiment was made. Under suitable conditions a bright spot can be observed at thecentre of the shadow of a three-penny bit. The coin may be sup-ported by three or four very fine wires, and its shadow thrown by. sunlight admitted at a pin-hole aperture placed in the shutter of adarkened room. The coin may be at a distance of about fifteen feetfrom the aperture, and the screen at about fifteen feet beyond the obtain a more convenient illumination, a larger aperture in theshutter may be filled by a short focus lens, which forms a diminutiveimage of the sun ; this image serves as a point source of light. Asmaller disc has some advantages. Fig. 1 is reproduced from a photo-graph of the shadow of a silver penny piece, struck at the time of theCoronation. The shadow, formed in the manner just described, wasallowed to fall directly on a photographic plate; after development anegative was obtained, in which the dark parts of the shadow wererepresented by transparent gelatine, while the bright parts wererepresented by opaque deposits of silver. To obtain a corr
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Keywords: ., bookauthorroyalins, bookcentury1800, bookdecade1850, bookyear1851
