368 OPTICAL PROJECTION 



geometrical design prepared in mica, the whole cemented 

 down as one slide. Inserted in the stage with the A plate 

 next the polariser, this plate has no effect, 1 and the ordinary 

 complementary colours appear ; but with A next the analyser, 

 the rotational colours appear. Mr. Fox's slides were pieced 

 together as a mosaic, the portions being cut out of different 

 thicknesses; but I have produced much better effects by 

 designing patterns which could be built up by successively 

 smaller and smaller films, each laid down in the centre. This 

 construction gives a better gradation of colour in the different 

 parts of the design, and some of the tints thus obtained are 

 magnificent. For use superposed on his wedge, the same 

 gentleman prepared a quarter- wave in two halves as at B, the 

 effect of which is that in the upper and lower halves the 

 colours pass along the wedge in contrary directions. 



I myself devised what I think are still more beautiful 

 effects, by reversing alternate sectors of the quarter-wave, as 

 at c D E. Either of these superposed on a concave selenite, 

 or on concentric rings or squares of mica, causes simultaneous 

 expansion and contraction of the bands in adjacent sectors. 

 By placing underneath the colour-design an even film in a 

 rotator, and rotating this slowly aa the analyser is rotated, 

 the foundation colours themselves are beautifully varied, as 

 already seen, and the result is a kind of optical chromatrope 

 of great fascination. The eight -sector plate shown at E is most 

 suitable for concentric squares. Or any suitable geometric 

 design (i.e. some kind of eight-pointed or four-pointed star) 

 may be laid down on such a plate as E, and have a single plate 

 A superposed on it ; then if the analyser is rotated it will be 

 as if E were not there ; but if the polariser is rotated, the 

 contrary sector rotations of E will come into play. If E be 

 superposed upon an even colour-film, simple contrary rotations 

 in adjacent sectors will be seen. 



1 Unless, as in the preceding note, the analyser be kept stationary and tha 

 polariser is rotated instead. 



