Light; a course of experimental optics, chiefly with the lantern . hespectrum band; but this need not matter to us. Arrangethe loose focusing lens f (the longest-focus one if thereare two) so as to focus the slit on the screen if the beamwere direct (the lantern must, of course, be deflected, asfor all prism work), and adjust the prism p otherwise asbefore. The whole arrangement is shown in Fig. 82, andits object is simply to produce on the screen the spectrum A brass cap with adjustable slit is, of course, much more convenient LIGHT. [chap. ot a slit upon which we can more readily make variou
Light; a course of experimental optics, chiefly with the lantern . hespectrum band; but this need not matter to us. Arrangethe loose focusing lens f (the longest-focus one if thereare two) so as to focus the slit on the screen if the beamwere direct (the lantern must, of course, be deflected, asfor all prism work), and adjust the prism p otherwise asbefore. The whole arrangement is shown in Fig. 82, andits object is simply to produce on the screen the spectrum A brass cap with adjustable slit is, of course, much more convenient LIGHT. [chap. ot a slit upon which we can more readily make variousexperiments. 69. Absorption of Colours —Providing now somecoloured glasses, or some strips of coloured gelatine betweenglass plates, we make some experiments which teach us avery important lesson. We are apt to think that the sunlightwhich comes through a red glass window is all turned intored—made red. Well, there is the spectrum of our com-plete or white light on the screen, drawn out into its con-stituent colours. Over half the slit hold a bit of the red. Fig. 82.—Spectrum Work. glass; il the light, or most of it, is really reddened, allthe spectrum ought to be turned into red. It is no suchthing, however. There is no colour in the spectrum of theglass where that colour does not exist in the ordinary spec-trum ; the sole effect is that certain colours are cut out, orabsent. We get the colour, as so often before, by suppressingcolour. If the glass is a pretty pure red, only red and aHttle orange, a b, is seen in the spectrum of the half shtcovered by the glass; all the rest is cut away. So of all VI,] ABSORPTION. 113 the other gelatines or glasses, but we soon find it is verydifficult to find a pure colour; generally there are left, atleast, two well-marked colours ; and if we unite just thoseportions of the ordinary spectrum, by employing properslits in proper places, and uniting the colour passing throughthem by our confectioners jar, or a cylindrical lens (§ 49)we
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