Optical projection: a treatise on the use of the lantern in exhibition and scientific demonstration (1906)

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372 OPTICAL PROJECTION intermediate points where the analyser is crossed. If we place a plate of selenite in the stage, say red and green, we shall have red at two opposite points, green at two opposite intermediate points, and no colour at the four points of 45°, all graduating smoothly. If we use in the stage a quartz cut transversely to the axis, we shall have the rotational colours, as a spectrum round the ring, twice repeated. We have simply what, in the ordinary way, would be successive appearances of the image of the aperture, simultaneously appearing round a ring. Mach further added to his apparatus, next to the deflecting prism, a direct-vision dispersive prism, so placed that its dispersion is also in the plane of polarisation of the analyser ; whilst a slit is attached to the latter, instead of the square aperture. Thus in any given position we have a radial spec- trum, whose violet end should be towards the centre to equalise the fainter light; and on rotating the whole arrange- ment, we now have as the foundation a circular spectrum, with the violet towards the centre. If we place in the stage a plate of crystal too thick to show colour in the ordinary way—say 1 to 2 mm. thick—our ring spectrum will be con- tinuous in the two azimuths of 45°, but will be crossed by interference bands (appearing on the ring as concentric arcs) along the horizontal and perpendicular diameters, while the dark bands in the one diameter will be opposite the bright coloured bands in the other. Finally, Mach places in the stage a plate of quartz cut transversely to the axis—say 8 mm. thick. We have already seen (§ 214) the gradual passage of the interference bands, on rotating the analyser, along the spectrum of such a plate. With the revolving apparatus it is manifest that this must be translated into a circular spectrum traversed by black bands in the form of beautiful continuous bold spirals. This last experiment is one of peculiar beauty. 216. Bi-quartz Effects.—Crystals of quartz being found