POLARISED LIGHT 



367 



from these, the colours change, in contrary orders in each 

 half, from one end of the spectrum to the other. 



213. Rotational Colour Experiments. These phenomena 

 open the way to singularly beautiful experiments. If the 

 quarter-wave plate is superposed upon a selenite or mica 

 wedge, the bands of colour will appear to move across the 

 screen. 1 Superposed upon the concave selenite, as the analyser 

 rotates the rings expand or contract, with fine effect. If 

 the quarter- wave is used in a rotator, it will be shown that 

 after rotating it 90 the order (expansion or contraction) is 

 reversed. Or if the analyser be kept stationary, and the 

 quarter-wave itself is continuously rotated, the rings alter- 

 nately contract and expand, as the quarter-wave is gradually 

 added to, or subtracted from, the thickness of the rings. 



The exact nature of what takes place is best shown by 

 spectrum analysis. Placing a slit in the stage with a selenite 



FIG. 203. Quarter-wave Preparations 



or mica thus circularly polarised by the rotational method, 

 and interposing a prism (direct prisms are far the best for 

 these experiments) the colours destroyed by interference 

 appear as dark bands in the spectrum ; and as the analyser 

 rotates, the bands travel along the spectrum. On reversing 

 either quarter- wave film or colour-plate, the direction of this 

 motion reverses. 



Mr. Fox demonstrated these phenomena by superposing 

 a quarter-wave plate with its axis as at A (fig. 203) on a 



1 In all rotational colour experiments it is understood that the quarter- 

 wave comes between the pattern- plate and whichever part of the polarising 

 apparatus is rotated. It will act similarly next the polariser, provided this 

 latter is rotated instead of the analyser. 



