File:The London encyclopaedia, or Universal dictionary of science, art, literature, and practical mechanics, comprisiong a popular view of the present state of knowledge (1837) (14801858793).jpg

Identifier: londonencyclopae16lond (find matches)
Title: The London encyclopaedia, or Universal dictionary of science, art, literature, and practical mechanics, comprisiong a popular view of the present state of knowledge
Year: 1837 (1830s)
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Publisher: London : T. Tegg & Son
Contributing Library: University of California Libraries
Digitizing Sponsor: Internet Archive

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ng thus given a brief detail of theexperiments of Gmelin and Rittenhouse we shallproceed to explain more minutely the principleson which this illusion depends. It will after-wards be seen that inverting telescopes and mi-croscopes are not necessary to the production ofthis illusion ; but it may be best seen by viewingwith the eye-piece of an achromatic telescopethe engraving upon a seal when illuminated eitherby a candle or the window of an apartment.This eye-piece inverts the objects to which it isapplied like the compound microscope, and theexcavations or depressions of the seal are imme-diately raised up into elevations like a cameo, ora bas-relief. The cause of this illusion will beunderstood from fig. 7, where A represents aspherical cavity illuminated by a candle C. Theshadow of the cavity will of course be on theleft side S, and, therefore, if we view it throughan inverting eye-piece or microscope, the cavity V01»16.PA0E 153 . />>/. 7. DPTIC § PLATE T. -F^^.je, Fi\o.l5.
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I.ojuion/Juflufhedy bv TThctnut^- Trj/ir. /■.■,(^uM^M-ide/. J.Shmy sculp. OPTICS. 23! H-ill be seen as at A, fij. 8, with its shadow onthe right hand S of the cavity. As the candle Cremains where it was, the observer instantly con-cludes that what was formerly a cavity mast nowbe a spherical elevation or segment of a sphere,as nothing but a raised body could have its sha-dow on the right hand S. If a second candle isnow placed on the right hand side of A, so thatit is between two candles, and is equally illumi-nated by both, the elevation w,ll again sink intoa cavity, as in fig. 9. 294. If the object A, in place of being a ca-vity, is actually the raised segment of a solidsphere, the same phenomena will be observed,the inverting eye-piece converting it into acavity. These two experiments may be mademost successfully with a seal, and an impressiontaken from it. 295. It cannot therefore be doubted that theoptical illusion of the conversion of a cameo intoan intaglio, and of an intag

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