370 OPTICAL PROJECTION 



is to prepare two similar double wedges, the thicknesses of 

 mica successively increasing from each end, so that the 

 thickest band is in the centre. The bands should be about 

 iV ^ s\r inch wide, and as long as possible. Crossed, these 

 of course give a very beautiful floor-cloth kind of pattern, 

 with either the colours of the added films, or two diagonals of 

 black squares. On superposing a plate of A, B or c (fig. 203), 

 c being prepared not only with the quadrant sectors as drawn, 

 but another plate with the sectors on lines arranged diagonally, 

 most extraordinary differences in the phenomena will ba 

 observed. 



The most beautiful of all the mica preparations I have de- 

 vised, however, so far as spectacular effect upon the screen is 

 concerned, are composed of two geometrical designs, each cir- 

 cularly polarised by a quarter-wave film, aud then superposed. 

 The two may be similar, or different ones designed in relation 

 to each other, such as a design composed of straight lines 

 with one composed of circles or curved lines. The first is of 

 course circularly polarised in the ordinary way; but these 

 rotational colours are again resolved, recompounded, and 

 again circularly polarised by the films in the second design ; 

 and the result is a transition and play of colour unrivalled 

 in magnificence, and apparently inscrutable till its components 

 are analysed. This is not all. If every now and then the 

 original polarising plane be rotated a little, the relations of 

 all the axes are altered, and so are the colour variations, 

 until when the polariser stands at 45, the first of the two 

 patterns (being in the same plane) becomes quite obliterated, 

 and only the second single design is left upon the screen. 



214. Quartz Rotation. A plate of quartz cut transversely 

 to the optic axis exhibits the same rotational change of colour, 

 showing that plane- polarised light is resolved in its interior 

 into two circular waves. The best thickness for an apparently 

 complete range of colours is 1\ mm., which also gives the 

 'transition-tint' between first and second orders. If an 



