THE MICROSCOPE BY MEANS OF THEIR OPTICAL CHARACTERS. 609 



the slot must evidently in this case also be slow ( -f- ). If on 

 the other hand the slow direction of the wedge correspond with 

 the fast direction in the crystal section and vice versa, the resulting 

 relative retardation will be equal to the difference of relative re- 

 tardations in the two, and they may be said to be in the subt) 'active 

 position (fig. 3). In this case the direction in the crystal section 

 parallel to the length of the wedge and therefore to the slot will 

 be fast ( ). If the relative retardation of the crystal section be 

 within the limits of relative retardation shown by the wedge, 

 there will, as the wedge is advanced through the slot in the 

 subtractive position, be ultimately seen a black band traversing 

 the crystal at right angles to the length of the wedge. This 

 marks the point where the relative retardation in the wedge 

 exactly neutralises that in the crystal section, being equal to it 

 but opposite in character. The relative retardation shown in 

 the graduation of the wedge at the point where the black band 

 appears must therefore be that of the section also. 



If the mineral gives rise to very high relative retardation and 

 shows only pale pink and green tints or the white of the higher 

 orders, except on the margin where bands of the lower-order 

 colours are visible, the character of the section may most easily 

 be determined by noticing how these bands move when the 

 wedge is inserted. If they move inward from the margin, the 

 mineral and the wedge are in the subtractive position ; if out- 

 wards towards the margin, in the additive position. In such 

 cases it is frequently desirable to employ an especially thick 

 wedge with a comparatively large angle. It sometimes happens 

 in the case of minerals with high birefringence that, even when 

 the wedge is inserted in the subtractive position and the relative 

 retardation at its thick end exceeds that of the mineral, no 

 definite black band can be recognised, but when the wedge is 

 inserted up to a certain point, irregular lines appear, which 

 are too thin for the colours to be recognised, and when the 

 wedge is pushed still farther in, they disappear. The mean of 

 the values of the relative retardations of the quartz wedge at 

 the points where these lines appear and disappear may be taken 

 as that of the mineral under examination. 



In all cases of difficulty in making this determination it is 

 best to use strictly parallel light. 



With strongly pleochroic minerals the black band does not 



Jourx. Q. M. C, Series II. No. 77. 34 



