188 ON THE SPECTEA FORMED BY THE PASSAGE OF POLARISED LIGHT, ETC. 

 y" remaining the same. We may put these values into the form 



x" — c cos ( nt + f j cos| 



y" — c cos ( nt + ~ j sin p 



2 



This shows that after emerging from the circular polarising film the ray is 



plane-polarised, that the plane of polarisation inclined „ p to that of primitive 



polarisation. 



If the emergent light is analysed by a dispersion prism, and a Nicol's prism 

 inclined (B to the plane of primitive polarisation, there will be black bands 

 (perfectly black) for all colours for which 



p = 2/3 or 2/3 + 2nv , 



and as the prism is turned these bands will march forwards in a regular manner 

 across the spectrum. 



This very beautiful experiment, in which the phenomena of rotatory polarisa- 

 tion are imitated, is not so well known as it deserves to be. One form of it is 

 due, I believe, to Biot, and another to Wheatstone, but the arrangement here 

 described is by far the most convenient. 



When the second plate is thick, then for some points of the spectrum its 

 retardation is (2 n + £) it. At these points the bands will move forwards when 

 the analyser is turned. At an intermediate set of points the retardation is 

 (2n— ^)ir. At these points the bands will appear to move backwards. At 

 intermediate points the retardation is nw. At these points the bands will not 

 move, but will become deeper or fainter. I suppose this to be the explana- 

 tion of the experiment described at p. 181, but the arrangement of the films 

 is not very precisely described. 



The experiments with the rings in crystals are very well described, and 

 must be beautiful, but are not so instructive to a beginner as those with the 

 selenite plates. Those, however, who have made out the meaning of the expe- 

 riments first described have a good right to regale themselves with gorgeous 

 entanglements of colour. 



