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



gypsum plate (which should agree with its previously ascertained 

 value) and half the difference that of the crystal section. If 

 the gypsum plate and quartz wedge are to be used together, 

 the former should be inserted in the lower slot, leaving the upper 

 for the latter ; the upper nicol would then be necessarily placed 

 above the eye-piece. 



F. E. Wright has devised a useful combination of quartz wedge 

 and gypsum plate,* and I have employed the same idea in the 

 following manner (fig. 5). A quartz wedge is superposed on 

 a gypsum plate showing the sensitive tint, both being con- 

 structed with the usual orientation (see above), so as to leave 

 beyond the thin end of the wedge a square of gypsum which 

 may be used as an ordinary gypsum plate. The quartz will 

 show a black band where it exactly neutralises the gypsum, 

 and the same succession of colours in opposite directions from 

 this point, which is indicated by a line marked zero ; but 

 those on one side stop short a little before the colour of the 

 plate is reached. Every hundred micro-millimetres of relative 

 retardation on either side is shown by graduations. If the 

 direction of the crystal section parallel to the slot be fast ( ) 

 the black band will move towards the thick end of the wed;e, if 

 slow ( + ), towards the thin end. 



Mica Steps (fig. 6) consist of a succession of narrow cleavage 

 plates of muscovite with their length cut parallel to the trace 

 of the optic axial plane and therefore slow. Each strip should 

 have a relative retardation of a hundred micro-millimetres. 

 They are of different lengths, and when superposed form a suc- 

 cession of steps each large enough to cover the whole cone of 

 light in the lower slot, where they are usually employed, though 

 they are equally useful in the focus of the eye-piece, if the upper 

 nicol be placed above them. In either case they show a dis- 

 continuous series of colours corresponding to differences of one 

 hundred micro-millimetres. If they are inserted over a crystal 

 section it is easy to see whether the two show additive or sub- 

 tractive relations. In the former case the stage should be rotated 

 till the fast direction of the crystal section is parallel to the slot. 

 It may then happen that the crystal section is exactly neutralised 

 by one of the steps and must therefore have the same relative 



* Journal of Geology, vol. x., 1902, pp. 33-35. See also Min. 

 Petr. Mitt. (Tschermak), vol. xx., 1901, pp. 275-6. 



