Elements of natural philosophy (Volume 2-3) . same side of the axis OF; in like manner, if DFbe the next consecutive deviated ray to D F\ it willintersect this atter in same point as c\ and so forother deviated rays up to that one which coincides with theaxis. The locus of these intersections c\ e, &c, is calleda caustic curve / and if the curve be revolved about theaxis 0 F, it will generate a caustic surface. This surfacewill spring from the focus of the axial rays at F\ as avertex, and open out into a trumpet-shaped tube towardsthe deviating surface. The deviated wave will no longer be sphe
Elements of natural philosophy (Volume 2-3) . same side of the axis OF; in like manner, if DFbe the next consecutive deviated ray to D F\ it willintersect this atter in same point as c\ and so forother deviated rays up to that one which coincides with theaxis. The locus of these intersections c\ e, &c, is calleda caustic curve / and if the curve be revolved about theaxis 0 F, it will generate a caustic surface. This surfacewill spring from the focus of the axial rays at F\ as avertex, and open out into a trumpet-shaped tube towardsthe deviating surface. The deviated wave will no longer be spherical, but willbe of such shape that its section d d d o\ by a planethrough the axis of the deviating surface, will be the in-volute of the section c c F\ by the same plane, of thecaustic surface, taken as an evolute. If after deviation the wave approach the caustic, the latter will be real,Flg37 being formed by the doubling over,as it were, of thedeviated wave up-on itself, thus pro-ducing at the cuspc double the ethe-real agitation due. ELEMENTS OF OPTICS. 219 to either segment Fz d or cf ci separately. If, on the con- virtual canstictrary, the wave recede from the caustic on being devia-ted, the caustic will be virtual. Caustics are finely illus-trated on the surface of milk when the li^ht is reflectedupon it from the interior edge of the vessel in which Illustration-it is contained. § 58. We have only spoken of a pencil of light whose 0blique is on the axis, which is usually called a directpencil. When the radiant is off the axis, the axial rayof the pencil becomes oblique to the deviating surface,and the pencil is said to be oblique. In the case of anoblique pencil, however small, the deviated rays will not,in general, meet the axis as in the case of the directpencil, but will all intersect two lines at right anglesto each other and not situated in the same plane. Theselines are called focal lines, and the property of the de-Focal lines;viated rays by which
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