. The microscope and its revelations. °angle, composed of glass having twice the dispersive power of theformer. Dispersion will be manifestly destroyed, because it is equalin amount and opposite in nature to that possessed by the prism of50° ; but the prism with an angle of 25° will not neutralise all therefraction effected by the prism of 50°. These conditions plainly suggest the solution of the problem, for] art of the convergence is maintained while the whole of thedispersion is destroyed. The spherical lenses which answer to these prisms area cro\biconvex, fitting into a flint plano-concav
. The microscope and its revelations. °angle, composed of glass having twice the dispersive power of theformer. Dispersion will be manifestly destroyed, because it is equalin amount and opposite in nature to that possessed by the prism of50° ; but the prism with an angle of 25° will not neutralise all therefraction effected by the prism of 50°. These conditions plainly suggest the solution of the problem, for] art of the convergence is maintained while the whole of thedispersion is destroyed. The spherical lenses which answer to these prisms area cro\biconvex, fitting into a flint plano-concave of double the dispersivepower It has been pointed out above that all the other colours lie intheir proper order between the rays II and V (tig. .>). Let us selectone. green, and represent it by (!. Now if G lies midway betweenII and V in the prism of 50° of angle, and also between Iv and Y inthe prism of 25° of angle, its dispersion will also be means that when the dispersion between the three colours in. FIG. 20.—Eecomposition of light by prisms. (Fromthe Forces of Nature.) ACHROMATIC OBJECTIVES 19 one kind of glass is proportional to their dispersion in the other,then when any two are destroyed the third is destroyed with unfortunately is not the case in practice, because two kinds ofglass having proportional dispersion powers cannot be , however, is what really happens. G may lie midway betweenR and Y in one kind of glass, but in the other it may lie, forinstance, much nearer R, say a third instead of half the distanceof R from Y. If now the dispersion of R Y be destroyed. G willbe left outstanding. If a different angle of prism be chosen, so thatR and G are neutralised, then Y must be left outstanding. This want of proportion in the dispersion of the various coloursof the spectrum in two kinds of glass is termed the irr<itiii<iHti/ ofthe spectrum, and the colour or colours left outstanding in a correctedcombination
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Keywords: ., bookcentury1900, bookdecade1900, booksubjectmicrosc, bookyear1901
