304 Mr. L. Wright on Optical Combinations 



uniform colour, on rotating the analyzer we get, as you see, 

 the contrary quartz rotations. But it lately occurred to me 

 that a still more beautiful demonstration of these rotational 

 colours would be obtained by another combination, which 

 deserves perhaps to be called an " optical chromatrope." We 

 place first in the stage next the polarizer a large even-tint 

 film in a rotating frame; next to that a concave selenite plate 

 showing Newton's rings; next to that again our quarter- wave 

 plate in sectors. As we rotate the analyzer, one set of alter- 

 nate sectors of the rings approach the centre, while the inter- 

 mediate sectors recede from it ; and if we now at the same 

 time rotate the even-tint plate, we simultaneously vary the 

 colour phenomena in an exquisitely beautiful manner. 



A|X plate divided into four sectors or quadrants, with their 

 planes alternately reversed in the same way (fig. 5), enables 

 us to demonstrate the nature of the curious modifications of 

 the rings and brushes in a plate of crystal when circularly- 

 polarized convergent light is employed. Here, for example, 

 are the rings and cross of calcite: interposing a £\ plate, the 

 black cross disappears into a grey nebulous one, and on oppo- 

 site sides of each arm the quadrants of rings appear dislocated, 

 the dark rings of one quadrant opposing the light rings of its 

 neighbours. Interposing another J\ plate on the other side, 

 on rotating the analyzer one opposite pair of quadrants con- 

 tracts while the intermediate ones expand, so that in two com- 

 plementary positions we have unbroken circles. The same 

 phenomena precisely are exhibited by this disk of chilled glass 

 in parallel light, the gradually decreasing elasticity of the 

 glass as we recede from the centre having the same effect as 

 the increasing convergence of the rays has in the calcite. 

 Now it is pretty easy to explain this phenomenon to a student 

 by such a diagram as this (fig. 6) representing our crystal or 

 glass with the Nicols crossed. The circularly-polarized ray 

 we know is, on entering the glass, decomposed into its two 

 plane-polarized components, of which one (let us suppose that 

 denoted by the arrow-heads) is retarded a quarter of a wave. 

 But the calcite or glass, beside this, itself also retards either 

 the radial or the tangential vibration more than the other 

 component — in calcite the radial. Taking, then, any originally- 

 circular ring caused by the calcite retardations alone, we see 

 that in two opposite quadrants the ^X plate retards the radial 

 vibrations a further quarter-wave, while in the alternate 

 quadrants it accelerates them a quarter-wave. The result must 

 obviously be a half- wave dislocation. As I have just observed, 

 such a diagram sufficiently explains it all; but it seems to me 

 better actually to represent it optically, by introducing the 



