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 



