Petrographic Microscope. 425 



1-2°. If only one optic axis appears within the field, an error 

 of ±3° is possible on even a favorable section. 



(c) Plate g (fig. 1 and fig. 10a and b), bi-quartz wedge plate for 

 the accurate determination of extinction angles. — The principles 

 and methods for determining extinction angles are presented at 

 length in another paper,* where it is shown that for general 

 purposes with variable light conditions and varying sensitive- 

 ness of the eye, a satisfactory general method should possess an 

 element of adjustable sensibility in order to meet best the dif- 

 ferent conditions. This was found to be realized most satisfac- 

 torily in the bi-quartz wedge plate. The present plate was 

 made after the following specifications : (fig. 10a) Wedge of 

 right-handed quartz, length 35*3 mm , width 6 mm , thickness at 

 thin end 35 mm , at thick end -85 mm ; plate of left-handed quartz, 

 length 35*3 mm , width 6 mm , thickness -4 mm ; thin plate to be 

 cemented on the wedge to a combination plate wedge which 

 gives zero extinction at a distance 3'5 mm from the thin end. 

 The same specifications to be followed with a wedge of left- 

 handed quartz and a plate of right-handed quartz, likewise 

 superimposed and cemented side by side as indicated in the 

 figure and in such a way that the line of total extinction in the 

 first combination is the extension of the line of zero extinction 

 in the second (C in fig. 10b). This wedge in sodium light 

 gives at the extreme end symmetrical extinction of about 

 ±10° (fig. 10b), while at the thin end it is ±1°. 



The position of darkness between crossed nicols for any plate 

 is tested by simply inserting the bi-quartz wedge plate and 

 observing the effect on the adjacent halves of the plate. If 

 the position of total extinction coincides precisely with the 

 planes of the nicols, both sides of the wedge will show the 

 same intensity of illumination on insertion. If this is not the 

 case, the observed plate must be revolved again and the test 

 repeated. With this method on favorable sections the position 

 of total extinction can be determined on a single trial within 10'. 



The determination of an extinction angle on a mineral plate 

 involves two distinct steps — the location of the exact position 

 of total extinction and the angular relation (optical system, 

 accurately centered) of this direction to some observed crys- 

 tallographic direction, as crystal edge or cleavage line. The 

 error of the latter determination is not great since the eye is 

 sensitive to errors in parallelism of adjacent lines as cross hair 

 line and cleavage line. The measurement of the extinction 

 angle by simply revolving the stage and thus determining the 

 position of total extinction, is sufficiently accurate for ordinary 

 purposes, if the average of a number of determinations be 



*On the Measurement of Extinction Angles in the Thin Section, this 

 Journal (4), xxvi, 349-390, 1908. 



