POLARISED LIGHT 369 



By superposing upon a colour-film cut into strips which 

 are reversed, a quarter-wave plate also cut into strips reversed, 

 the strips crossing the others at right angles, and blacking out 

 the margins of the preparation, a chess-board pattern can be 

 made without any great difficulty, which shows contrary 

 rotations in adjacent squares. 



Very beautiful effects are also produced by superposing D 

 on E, or a quarter-wave made in twelve sectors, upon a square 

 of chilled glass, 1 in either of its principal positions. 



I have also constructed a design built up as a kind of 

 circular wedge, the circle being divided into twenty-four sectors, 

 each one in succession being one film thicker than its pre- 

 decessor. When a single quarter-wave is superposed upon 

 this, the result is of course an apparent revolution of the 

 colours round the centre ; but this is still better 

 shown by making the quarter-wave of two con- 

 centric parts with axes reversed as in fig. 204, 

 when the outer portion apparently revolves in the 

 contrary direction to the inner portion. If the cir- 

 cular wedge itself is crossed (as regards axes) upon 

 a preparation of circular rings, or a concave selenite, the 

 result is a series of beautiful black spirals starting from the 

 centre ; and upon these, again, fig. 204 may be superposed. 

 Again, if a sector-plate in its depolarising position (axes at 45) 

 be placed in the stage as a foundation, and fig. 204 be super- 

 posed, the alternate areas resulting will be black and white in 

 contrast ; and in whatever position the second plate be placed, 

 on rotating the analyser to a corresponding position, this 

 effect will be seen. Hence the operation of Professor S. P. 

 Thompson's quarter-wave rotator for the polarising plane 

 (p. 347). 



A very instructive method of showing the remarkable 

 differences of result from superposing films in different ways, 



1 For details see the paper in Proceedings of the Physical Society already 

 referred to. 



BB 



