Ocular refraction and the shadow test . heir axesand determination of species has been explained. Sphero cylindersand their characteristics; prisms, simple and in combination havebeen explained and the student who has faithfully followed up the 88 OCULAR REFRACTION. work SO far outlined need fear no difficulty in recognizing any kind oflens. The only point yet unexplained is the determination of theexact dioptric value of an unknown lens. The treatment of the subject will be taken up merely in the formof a review which will include this last point, the procedure to deter-mine the character and
Ocular refraction and the shadow test . heir axesand determination of species has been explained. Sphero cylindersand their characteristics; prisms, simple and in combination havebeen explained and the student who has faithfully followed up the 88 OCULAR REFRACTION. work SO far outlined need fear no difficulty in recognizing any kind oflens. The only point yet unexplained is the determination of theexact dioptric value of an unknown lens. The treatment of the subject will be taken up merely in the formof a review which will include this last point, the procedure to deter-mine the character and power or powers of an unknown lens will begiven To neutralize means to destroy J^ozcer. It is inferred that a certainpower or property is possessed by something when the term neutra-lize is used, to which by the act of neutralizing, is opposed a similarpower or property that counterbalances its action. To neutralize isthus to oppose power of a certain kind with similar power operatingin an opposite manner so that both are destroyed. {. Figure S9. In optics we have lenses that possess directly opposite powers,creating opposite effects, viz:—those lenses that converge light raysand those that cause them to diverge. To neutralize optically we oppose convex and concave poiiur anddestroy both, the effect being that of piano glass. To demonstrate thetheory of neutralizing let fiigure 89 represent two prisms of equalpower and similar form. If they should be imposed base to apex, itwill be seen by reference to III, figure 89, that across section throughwould show the surfaces A. B. and F. E. to be parallel and thereforethe optical effect would be that of piano glass. The refractive powerof each is destroyed by so opposing their powers. To prove this by LENSES. gg experiment take a prism of any power and look through it at a verti-cal straight line, holding the prism as shown by I, figure 87, the linewill appear broken. If a prism of equal strength be now imposedthe line will assume
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