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



to the optic axis in uniaxial crystals, and to the optic axial plane 

 in biaxial crystals. The actual birefringence in a section may 

 be anything between this and zero. 



The maximum birefringence in millesims may be obtained 

 from the value in homogeneous units found in text-books by 

 moving the decimal point three places to the right. 



The Quartz Wedge. If the relative retardation is to be de- 

 termined at the same time as the character of the directions 

 of vibration, a quartz wedge or mica steps must be employed. 

 The quartz wedge is cut in this country with its length parallel 

 to the optic axis, which is the direction of vibration of the light 

 propagated with the least velocity. The length is therefore 

 slow ( -f- ) while the width is fast ( ). As wedges are some- 

 times cut in different directions, the character of the length 

 should be engraved on the glass as shown in fig. 3. 



The wedge should be graduated so as to indicate the relative 

 retardation at different points (see fig. 3). It should be inserted 

 in focus (see p. 599), otherwise the colours will be blurred from 

 overlapping and the graduation be invisible. 



If the wedge be inserted in the slot between crossed nicols, 

 when there is no birefringent mineral in the field or none which 

 is not in the position of extinction, the normal succession of inter- 

 ference colours is seen commencing at the thin end of the wedge, 

 where, however, the black and darker grey are usually missing 

 on account of the difficulty of preserving the thin end from 

 abrasion. 



If, however, there is a birefringent mineral present in the 

 diagonal position, so that the directions of vibration of the light 

 traversing it are parallel and at right angles to the slot, and 

 therefore parallel to those of light traversing the quartz wedge, 

 the relative retardation of light traversing both the mineral and 

 the wedge will be the combined effect of the relative retardation 

 in each. 



If the directions of the slow ( + ) and fast ( ) vibrations 

 respectively in the mineral are the same as in the quartz 

 wedge, the colour seen at any point where the two are superposed 

 will correspond to a relative retardation equal to the sum of the 

 relative retardations of both. This may be referred to as 

 the additive position. As the length of the quartz wedge is slow 

 ( + ), the direction in the crystal which coincides with that of 



