IO2 METHODS OF PETROGRAPHIC-MICROSCOPIC RESEARCH. 
The wedge or birefractometer of Amman consists of a quartz wedge placed 
in the lower focal plane of the eye lens of a Huygens ocular. It is inserted by 
means of a micrometer screw until the interference tint of the mineral plate 
under investigation is exactly compensated and the path-difference thus 
obtained. In the Cdsaro wedge compensation is carried only to the sensitive 
tint either between crossed or parallel nicols. The Leiss modification of the 
Amman birefractometer covers only half the field and carries, in addition, 
a micrometer scale directly beneath the wedge, so that accurate settings can 
be made. On the upper surface of the graduated quartz wedge of Evans a 
scale is engraved which gives directly the path-difference of the emergent 
waves. The second wedge of Evans consists of two adjacent quartz wedges 
in combination, the direction of elongation of the one half being C, that of 
the second rt. This wedge is inserted until in the one half the interference 
color of the mineral plate is exactly compensated ; in this position the path- 
difference of the waves emerging from the second half is twice that of those 
from the mineral plate alone. The Siedentopf wedge consists of two wedges 
superimposed and with different ellipsoidal axes parallel with the elongation. 
Where both wedges have the same thickness a band of exact compensation 
occurs and the effect of the combination for that particular point is neutral. 
The wedge is infinitely thin at that point and increases in thickness on 
either side of the dark achromatic band. The wedge of the writer consists 
of a quartz plate and a quartz wedge after the manner of the combination 
wedge* and is so calculated and cut that the divisions on the scale read 
directly to 10 ^M. the zero line of the scale coinciding with the achromatic 
line. In principle all these wedges are practically identical with the Babinet 
compensator. They are inserted in the lower focal plane of a Ramsden 
ocular and require the use of a cap nicol. If the relative dispersion of the 
substance under examination be not greatly different from that of the quartz, 
the graduated wedge furnishes good results even in white light. By the use 
of the graduated wedge, the birefringence formula becomes y a = --, where 
6 = direct reading in nn from the scale on the wedge and d = thickness of the 
plate (in /u/u). 
If the interference color of the plate under observation is of a higher order 
than can be compensated by the wedge or Babinet compensator, a bire- 
fracting quartz plate of known path-difference can be inserted and the wedge 
thickened optically by just that much. In the actual determinations of 
the birefringence parallel beams of polarized light should be used to obvia ti- 
the disturbing effects of obliquely incident light. The condenser system 
should be lowered or, if possible, removed altogether. The effect of obliquely 
transmitted light-waves can be readily observed on any strongly birefract- 
ing substance where the interference color changes perceptibly on passing 
from central to oblique illumination.! 
In 1908,1 the writer described a combination wedge consisting of two 
adjacent combination quartz wedges so cut that the least ellipsoidal axis C 
*T.M. P. M..20, 375, 1901; Jour. Ceol., 10,33-33. 1902; E. Sommerfeldt has recently suggested (Zcitscbr. 
wtMen. Milcrosk.. 27, 448. 1910) that under certain condition* it would be an improvement if this wedge 
covered only half the field, a* in the Michel-L6vy comparator and the Lciss modification of the Amman 
birefractometer. 
tCotnpare). L. C.Schroeder vanderKolk. Zeitschr. wissen/Mikrcw.,8, 456-458. 1893; G.W. Grabham. 
Miner. Mag., IS, 340-341, 1910. 
;.Amcr. Jour. bd. (4). 2*, 37O-J7'. October, 1908. 
