DICHROISM 



places his eye close to the round aperture, a. An object-carrier, H, is sometimes provided 

 for the purpose of more conveniently holding the stone. This has the form of a brass 

 tube fitting loosely over the tube, h, and having the closed end perforated by an aperture 

 somewhat larger than the square aperture, b, over which the stone can be fixed with wax, 

 as shown, in the figure. This arrangement allows the carrier H, with the stone attached, to 

 be rotated while the calcite rhombohedron remains unmoved. Should this carrier not be 

 provided, the stone may be fixed by wax to a glass-plate, or simply held in the fingers in 

 front of the square aperture, and the instrument rotated in the hand. 



If the stone under examination is not dichroic, the two images o and e will be of the 

 same colour and will show no variation while the instrument or the stone is rotated through 

 360°. If, for example, a red garnet, which crystallises in the cubic system, is examined, the 

 two images o and e will both be of the same red colour as is the garnet itself when viewed 

 without the aid of the instrument. 



The images o and e given by a dichroic stone, on the contrary, will be in general 

 differently coloured. In four particular positions, however, at 90° apart, the colours of the 

 two images are identical. On rotating the stone or the instrument a difference between the 

 colours appears, which gradually increases and reaches a maximum at 45° from the original 

 position. Further rotation will again result in a gradual decrease in the colour difference, 

 and at 90° from the original position the colours of the two images once more become iden- 

 tical. The same changes occur during the rotation through the remaining quadrants, 

 and thus a complete rotation of 360° is accompanied by eight changes from identity of 

 colour in the two images to maximum difference in colour between them and vice versd. 

 The juxtaposition of the two images makes it possible to detect the smallest differences in 

 colour, and consequently the slightest degree of dichroism. 



We have previously seen that doubly refracting crystals are singly refracting along the 

 direction of an optic axis ; similarly dichroic crystals exhibit no dichroism in these 

 directions. To prove the absence of dichroism in a crystal it is therefore necessary to 

 examine it in two directions, or, as an additional precaution, in a third direction also. 

 After each observation the stone must be fixed on the holder, H, in a new position 

 and again rotated. The absence of dichroism can be conclusively proved only after an 

 examination of the stone in at least three different positions. The dichroism of a stone 

 may be so feeble that it is not possible to detect it even with the aid of a dichroscope ; 

 moreover, it must be borne in mind that a coloured doubly refracting stone is not 

 necessarily dichroic, and this feature is naturally absent in colourless doubly refracting 

 stones. The real or apparent absence of dichroism in a stone is therefore no proof of 

 its singly refracting character, but the presence of dichroism is, on the contrary, a conclusive 

 proof of the doubly refracting nature of the stone. 



The degree of dichroism in a crystal varies according to the direction through which 

 the crystal is observed. The colours of the two images seen in the dichroscope in the 

 examination of all dichroic stones become the more nearly identical as the optic axis of 

 the stone becomes more nearly coincident with the axis of the instrument. Conversely, the 

 greater the angle between the axis of the dichroscope and the optic axis of the stone the 

 more marked will become the difference in colour between the two images. The two colours 

 between which there is the maximum difference are known as the , principal or axial colours ; 

 these colours, as seen through the dichroscope, differ in tint from the colours the stone shows 

 when observed with the naked eye in the same direction. Uniaxial dichroic crystals, such as 

 tourmaline, show two principal colours, while biaxial crystals, such as cordierite, show three. 

 The pairs of colours, other than the axial or principal colours, shown by a precious stone 



