Volltext: Optical Projection: A Treatise of the Use of the Lantern in Exhibition and Scientific Demonstration

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. 208) 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.


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