Elements of natural philosophy (Volume 2-3) . encil, can, therefore, Equations (19) and °°*c*nl*ate °(20), be concentrated upon the ethereal molecules atanother place, as a focus, by the action of a spherical devi-ating surface : and the focus, whether real or virtual, be- & , 1 it . Aml the foons comes a source of light as well as the radiant, and is as becomes a source distinctly visible. When the focus is real, the deviated of lishtwave first becomes concentrated in, and subsequently 192 NATURAL PHILOSOPHY. whence the emanates from it; when virtual, the deviated wave prO-deviated wave ? *?
Elements of natural philosophy (Volume 2-3) . encil, can, therefore, Equations (19) and °°*c*nl*ate °(20), be concentrated upon the ethereal molecules atanother place, as a focus, by the action of a spherical devi-ating surface : and the focus, whether real or virtual, be- & , 1 it . Aml the foons comes a source of light as well as the radiant, and is as becomes a source distinctly visible. When the focus is real, the deviated of lishtwave first becomes concentrated in, and subsequently 192 NATURAL PHILOSOPHY. whence the emanates from it; when virtual, the deviated wave prO-deviated wave ? *? proceeds for real ceeds only from the deviating surface, but with dimen-and for virtual slons the same as though it had departed from the virtual foci. ° * focus. § 32. If the raywhich is deviated atthe first, be incident First deviated Upon a SeCOIld SU1*-nu* incident upon n -%ri -\r t ^ J . J face M Jy , having a second surface; 1 & a radius r\ andsituated at a dis-tance £, from thefirst, measured onthe axis, we may Fig. 18. suppose this ray to have proceeded originally from F1;and denoting the distance from the new vertex 0\ tothe point F\ in which this ray, after deviation at thesecond surface, meets the axis, by /, and the index ofrefraction of the second medium by m\ we shall havefrom Equation (20), Equationapplicable to thesecond deviation; w! — 1 / + m! r m! (/ + t) • • (21) Second deviatedray incident upona third surface;
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