A manual of photographic chemistry : including the practice of the collodion process . curvedimage by a biconvex Observe that the rayswhich fall upon the cir-cumference of the Lensare brought to a focusat a point nearer to theLens than those pass-ing through the cause of this is thesame as that alreadyshown to increase thechromatic aberration, viz. the greater inclination of the two * Curvature of the image is also caused by the oblique incidence of cer-tain rays of light proceeding from the object, that is, from the fact of theirnot falling upon the lens parallel to, but obl
A manual of photographic chemistry : including the practice of the collodion process . curvedimage by a biconvex Observe that the rayswhich fall upon the cir-cumference of the Lensare brought to a focusat a point nearer to theLens than those pass-ing through the cause of this is thesame as that alreadyshown to increase thechromatic aberration, viz. the greater inclination of the two * Curvature of the image is also caused by the oblique incidence of cer-tain rays of light proceeding from the object, that is, from the fact of theirnot falling upon the lens parallel to, but obliquely with regard to the axis. f This diagram is intended simply to illustrate the fact that the dif-ferent portions of the lens possess different refractive powers; bnt itmust not be taken to imply that the rays proceeding from the extremi-ties of the arrow Avould correspond only to the outside of the lens, sinceit has already been shown in a previous diagram that such is not thecase—that rays of light from every part of the object fall upon the wholecircumference of the ON THE NATURE AND PROPERTIES OF LIGHT. 55 surfaces of the Lens at the circumference than at any otherpart; hence, as it is at the surfaces of bodies that the deflect-ing power acts, it follows that the circumferential rays aremore powerfully bent in than the central rays. On the influence exerted on Spherical Aberration by the formof Curvature of the two surfaces of the hens.—Spherical aber-ration of Lenses is overcome in a manner very different fromchromatic aberration. Much in this respect depends upon theform of the lens, and it can be shown upon mathematical datathat a lens similar to that given in the following diagram—onesurface of which is a seetion of an ellipse, and the other of acircle struck from the furthest of the two foci of that ellipse—produces no aberration. At the earliest period of the- em-ployment of the Camera obscura, a\ Biconvex Lens was used to produce\ the image; but th
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