456 



Sir .1. (\>nrov on the Light reflected 



more intense in the light reflected by the crystals, and the 

 higher bands were more distinctly seen. 



The surface-colour of potassium permanganate, and the posi- 

 tion and intensity of the bands in the spectrum of the reflected 

 light, are independent of the relative position of the plane of 

 incidence to the long axis of the crystal, or to the striae pro- 

 duced by rubbing, when the powdered substance burnished on 

 glass is used. 



1. Siwface-Coloui's. — Freshly prepared surfaces of potas- 

 sium permanganate appear of a pale yellow when light, either 

 unpolarized or polarized in any plane, is incident upon them 

 at low angles. But with ordinary light, and with light pola- 

 rized in the plane of incidence, the amount of white light 

 reflected is so great at high angles that the surface-colour, if 

 any, is completely masked. 



When the incident light is polarized perpendicularly to the 

 plane of incidence, or when unpolarized light falls on the sur- 

 face and a Nicol is placed between the eye and the permanga- 

 nate, with its principal section in the plane of incidence, the 

 surface-colour is seen to change as the angle increases, becom- 

 ing successively green and blue, and finally white and metallic. 



The surface-colours alter with the surrounding medium. 

 The following Table gives, approximately, the colour at various 

 incidences, (A) when the light is either unpolarized or pola- 

 rized in the plane of incidence, and (B) when it is polarized 

 perpendicularly to that plane, for potassium permanganate in 

 air, tetrachloride and bisulphide of carbon. 



Angle of 

 incidence. 



Surrounding medium. 



30. | A 

 35. {^ 

 40. {^ 



MS 



60. j A 



M A 



CO. { A 

 65. {£ 

 70. {£ 

 75. {i 



Air. 



Tetrachloride of 

 carbon. 



Bisulphide of 

 carbon. 



Pale yellow. 



Pale yellow. 



Pale yellow. 



Pale yellow. 



White. " 

 Pale yellow. 

 White. 

 Yellow, with a 



green tinge. 

 White. 



Brilliant green 

 White. 

 Blue-green. 

 White. 

 Blue. 

 White. 

 Metallic, with 



blue shade. 



Yellow-green. ! Yellow-green. 



., ,, | Green. 



Yellow-green, i Yellow-green. 



,, ., Green. 



Yellow-green^ J Yellow-green. 



Green. j Green. 

 Yellow-green. Yellow-green. 

 Green. Blue-green. 



Yellow-green. Yellow-green. 



Bright green. Blue-green. 

 Green. Yellow-green. 



Blue-green. Blue-green. 



Green. 



Blue-green. 



Green. 



Blue-green. 



Greenish. 



Green. 

 Blue-green. 

 Green. 

 Blue-green. 

 Greenish white. 



