The London encyclopaedia, or Universal dictionary of science, art, literature, and practical mechanics, comprisiong a popular view of the present state of knowledge . vex lens, as aportion of a spherical surface, of which everypart is equally distant from the centre of thelens; but this estimate is extremely erroneous,for the effect of the obliquity of tiie diflereni 265 OPTICS. pencils of rays materially increases the curva-ture of the image. In fact no pencil of rays,falling obliquely on a spherical surface, can becollected any where to a perfect focus: theimage of a circle would become most
The London encyclopaedia, or Universal dictionary of science, art, literature, and practical mechanics, comprisiong a popular view of the present state of knowledge . vex lens, as aportion of a spherical surface, of which everypart is equally distant from the centre of thelens; but this estimate is extremely erroneous,for the effect of the obliquity of tiie diflereni 265 OPTICS. pencils of rays materially increases the curva-ture of the image. In fact no pencil of rays,falling obliquely on a spherical surface, can becollected any where to a perfect focus: theimage of a circle would become most distinct atone distance, and that of its diameter at another;but for both these images the surface ought to bemuch more curved than that which has beenusnally considered, and the mean of the cur-vature required for them, which must be thebest for the ground or bottom of a camera ob-scura is equal to that of a sphere of which theradius is three-eighths of the focal distance,when a double convex lens of crown-glass is em-ployed. 516. The general arrangement of this appara-tus may be best understood by a reference to theaccompanying diagram. Diverging pencils of. U n >^ h:. .-f r e ?? ~\^ \ >- a i^^-^ h •?-?. c C rays, as a 6 c, def, g h i, issuing from all pointsof the object A B, on the side next the convexlens E F, after passing through the lens, willconverge to as many points beyond it, and atthose points of convergent meeting they willform an inverted picture C D of the object on awhite paper. 517. Among the various phenomena of visionwhich were observed by the philosophers of the lastcentury those which arise from indirect impres-sions, and from the influence of highly attenuatedlight upon the retina, seem to have escaped theirnotice. 518. If we look at a narrow slip of white pa-per placed upon a black or a colored ground, itwill never appear to vanish, however long andattentively we view it. But if the eye is fixedsteadily upon any object within two or threei
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