286 



SUMMABY OF CURRENT RESEARCHES RELATING TO 



Mode of using the Quartz Wedge for estimating the Strength of 

 the Double-Refraction of Minerals in thin slices of Rock.* — Major- 

 General C A, McMahon describes a rough and ready method for esti- 

 mating the strength of the double-refraction of minerals in rock sections, 

 which he has used with advantage for some years. It serves to replace 

 the somewhat complicated methods, requiring special apparatus, of 

 Babinet and Michael Levy, when perfect esactness is not required. 



When a quartz wedge is inserted in a slot in the eye-piece of a 

 Microscope, arranged with crossed nicols, at an angle of 45° to the 

 plane of polarization, a series of chromatic bands will be observed in 

 the wedge, each band consisting of a spectrum of colours in an ascend- 

 ing order, the colours of the first order of Newton's scale being the 

 nearest to the thin edge of the wedge. The width of these bands varies 

 directly with the thickness of the quartz, and inversely with the slope 

 of the wedge. 



The stronger the double-refraction of a mineral, the higher will be 

 the order of the tint exhibited by it when slices of different minerals of 

 uniform thickness and at the same angle to an optic axis are examined. 

 The usual method of using the wedge therefore consists in comparing 

 the tint exhibited by the mineral with the corresponding tint in one of 

 the chromatic bands in the wedge. 



In working this method the author employs a special wedge (fig. 50), 

 which only occupies half the depth of the slot, so' that the observer is 



Fig. 50. 



1 2 3 4< S 



able to directly compare the tint of a mineral, say at d (fig. 50), with 

 the spectra seen in the wedge ahc. 



The method now to be described differs from the above in depending 

 on the phenomena produced in the wedge by the passage of light 

 through the mineral to the quartz. 



If, while the quartz wedge is inserted in the eye-piece as above, a 

 second quartz wedge be placed on the stage with its axis at right angles 

 to that of the wedge of the eye-piece, the velocity of the extraordinary 

 ray is retarded in one of the two plates and accelerated in the other. A 

 dark line will then appear due to the points where the velocity of the 

 extraordinary ray on emergence from the upper quartz wedge becomes 

 the same as that of the ordinary ray. 



If the analysing quartz wedge be kept stationary and the other 

 moved on the stage so that thicker and thicker portions of the quartz 

 are successively brought within the range of vision, the dark line moves 

 gradually from the thin towards the thicker end of the analysing wedge, 

 so that spectra (in inverse order) of the 1st, 2nd, 3rd, and higher orders 

 come in between it and the thin edge of the wedge. Thus the distance 



* Geol. Mag., v. (1888) pp. 548-53 (1 fig.). 



