358 Prof. Dove on the Dichrooscope. 



other, I have preferred to retain the same arrangement of the 

 apparatus as in the preceding case. 



\\ e shall most readily appreciate the phscnomena which now 

 appear, if that which is obtained in the calc-spar plate with a 

 certain-coloured light and with coincident planes of reflexion of 

 the polarizing and analysing mirror be considered as applied to 

 the phenomena which this plate exhibits when it is looked at in 

 light of another colour, the mirrors being crossed. The rings 

 are a combination of two coloured systems of rings with a light 

 and a dark cross, and which therefore appears brightly coloured 

 in the colour corresponding to the bright cross. The succession 

 of colours of the rings is consequently highly peculiar, and is 

 very distinct when well-annealed glasses are examined. If it be 

 desired to imitate the phenomena seen when a dichroite, dichro- 

 itic mica, a rubellite, repidolite, or smoky quartz is employed as 

 analysing arrangement, we have merely to use the proper 

 coloured glasses. In the first case the cross appears blue; in 

 the second it appears red with preponderating green rings ; in 

 rubellite the cross is the red of the alpine rose ; in repidolite 

 deep grey and almost black, as in dark smoky quartz, while the 

 rings appear brownish yellow. What we shall obtain by turning 

 the Nicol 90 degrees is manifest if we imagine that the pheno- 

 menon of the dark cross for one colour is changed into the light 

 cross, and, on the contrary, the phenomenon of the light cross 

 for the other colour is changed into that of the dark cross. 

 It is especially beautiful when an achromatic prism of calc-spar 

 is used as analysing arrangement with a red and blue glass. 

 The systems of rings, one with the blue and the other with the 

 red cross, then partially intersect one another. 



An idea is obtained of the colours which result from the com- 

 bination of the system of rings, if a longitudinal slit is inserted on 

 the side towards the eye, and this is viewed through a prism 

 of rock-crystal. By the intermixture of both spectra, colours 

 are obtained which would not be expected from the components, 

 corresponding to the investigations of Wiinsch and Helmholtz. 

 The active components are, however, obtained in the most differ- 

 ent intensities if a rotating Nicol is interposed between the prism 

 of rock-crystal and the eye. 



(7) The arrangement remains the same, but a rotating mica 

 plate of one- quarter difference of path is inserted at eh. 

 Right or left circular or elliptical light of one colour is seen to 

 combine with right or left circular or elliptical light of another 

 colour. The appearances here presented give a key to the com- 

 plicated phenomena seen in the polarizing apparatus when the 

 circular-polarizing mica or gypsum plate is changed for one 

 which has a much greater difference of path. 



