Light; a course of experimental optics, chiefly with the lantern . he other error is that tourmalines polarise by absorption. All that the absorption does is to take upor stop one of the already-polarised rays due to double refraction ; forif a very thin .wedge be ground, at the thinnest edge both images canbe distinguished. 214 LIGHT. [CHAP. slices of tourmaline, of such sizes and shapes that one canbe seen distinctly over the other. Let one be mounted inone of the 4 inches by 2^ inches wooden frames, and theother on a loose disc of glass, which can be secured in ametal circle by a spring, an
Light; a course of experimental optics, chiefly with the lantern . he other error is that tourmalines polarise by absorption. All that the absorption does is to take upor stop one of the already-polarised rays due to double refraction ; forif a very thin .wedge be ground, at the thinnest edge both images canbe distinguished. 214 LIGHT. [CHAP. slices of tourmaline, of such sizes and shapes that one canbe seen distinctly over the other. Let one be mounted inone of the 4 inches by 2^ inches wooden frames, and theother on a loose disc of glass, which can be secured in ametal circle by a spring, and rotated by a .pinion andcircular rack. Place both in the optical stage/parallel witheach other, and focus; then rotate the front one : the suc-cessive appearances are as in Fig. 134. When parallel, a,there is simply a rather deeper colour from the doublethickness; when the movable one is rotated 45, as at b, aconsiderable portion of light is stopped where both aresuperposed; when at right angles, c, no light whatever canget through—the screen there is B Fig. 134.—Two Tourmalines. It is plain we have here the same phenomena as before,only simplified by the absorption of one of the two prove it, we remove the fixed tourmaline, leaving onlythe rotating one in the stage, and placing with it a circularaperture in a plate or card, just large enough to encirclethe tourmaline. On the nozzle of the objective we placeone only >of the double-image prisms, which if of wideangle will quite separate the two circles of light with thetourmaline image in the centre ; if the separation is notsufficient for this, remove the second lens from the objective, Such rotating frames, of the standard 4 inches by aj inches size, canbe purchased for a few shillings of any good London optician, and atleast one is indispensable for many experiments. X.] POLARISATION BY REFLECTION. 215 .V and insert at c, Fig. i, a lengthening tube or adapter, about25 inches long, which, by reducing t
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