610 J. W. EVANS ON THE DETERMINATION OF MINERALS UNDER 



occur, as the excessive absorption of some or all colours in one 

 direction prevents the recombination of the vibrations to form 

 light vibrating parallel to the right and left cross wires. For 

 the same reason the interference colours of such minerals are 

 abnormal. As already stated, a similar result is obtained also 

 where the birefringence and therefore the relative retardation 

 varies considerably for different colours (see p. 606). In either 

 case it is, however, generally possible to estimate with a fair 

 amount of accuracy the central position from which the 

 relative retardation with its corresponding colours increases in 

 both directions. 



Pleochroic sections may be referred to as slow-dark and fast- 

 dark according as the character of the direction of maximum 

 absorption is slow or fast. The latter case is comparatively rare, 

 and when it occurs, as in aegyrine, riebeckite, arfvedsonite, 

 apatite, and andalusite, is of considerable diagnostic value. 



Similarly, where the colour varies considerably, crystal sec- 

 tions may be termed fast-red and slow-green, or as the case may 

 be. Where the contrast is between red on the one hand and 

 blue or green on the other, the fast vibrations are usually asso- 

 ciated with the former. 



In the double quartz wedge (fig. 4) which I described in the 

 Mineralogical Magazine (vol. xiv. (1905), pp. 91-2) there are two 

 wedges, one with the length slow ( + ) and the width fast ( ), 

 the other with these characters reversed. They have the same 

 angle and the same birefringence, so that when cemented by 

 Canada balsam side by side on a glass slip and inserted in 

 the slot between crossed nicols the colours stretch across the 

 two component wedges exactly as if they were one ; but if a 

 birefringent crystal section be in the field with its directions of 

 vibration parallel and at right angles to the slot, one side will 

 show additive effects and the other subtractive, so that the 

 existence of a small relative retardation is easily recognised, and 

 the amount of the relative retardation may be read off which- 

 ever direction of vibration in the crystal section is parallel to 

 the slot. It may be noted that the colour in one component 

 wedge opposite the black band in the other corresponds to a 

 relative retardation exactly double that of the crystal section 

 under examination. 



All forms of quartz wedge should be carefully calibrated by 



