Elements of natural philosophy (Volume 2-3) . et the axis in different points, depending upon the dis-tance of the point of incidence from the vertex. 190 NATURAL PHILOSOPHY. SMALL DIRECT PENCIL. Small directpencil; § 30. A pencil of light having its central ray coincidentwith the axis of the Fig. 16. M 11l in, i i i i i; / / Iff-, ; i i ji i i i i i i r*sj f r> s I i !\ i i i !?•••• i v\ L^r ! A. *J>^ deviating surface, iscalled a direct pen-cil; and • if such apencil be taken verysmall, the quantyz,in Equation (18),will be so small that General equation the products ofmade applicable w
Elements of natural philosophy (Volume 2-3) . et the axis in different points, depending upon the dis-tance of the point of incidence from the vertex. 190 NATURAL PHILOSOPHY. SMALL DIRECT PENCIL. Small directpencil; § 30. A pencil of light having its central ray coincidentwith the axis of the Fig. 16. M 11l in, i i i i i; / / Iff-, ; i i ji i i i i i i r*sj f r> s I i !\ i i i !?•••• i v\ L^r ! A. *J>^ deviating surface, iscalled a direct pen-cil; and • if such apencil be taken verysmall, the quantyz,in Equation (18),will be so small that General equation the products ofmade applicable whiell it is a faqtor may, without mate-rial error, be omitted. This will reduce Equation (18) to i i i \ WW to this case; Equation for asmall directpencil; (f-r).f = m.(f-r).f or / mvf (m — 1) .f + r (19) and taking the reciprocal, Reciprocal of thesame; 1 m — 1 1 T - H F mr m-J If f be con- Fig, : stant, or the M (20) 17. rays all proceedfrom the same Conclusion for a point F On tliesmall direct 7 r , pencil. aXlS W0™ de~. \ ±k \ \ \ \ \ _Ui I_ _L i_l _!_ /7 I I J I I I sP / \ I I I I I 1 I
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