8 BULLETIN 1108, U. S. DEPARTMENT OF AGRICULTURE. 



of the field of the microscope and only the arms of the cross are 

 visible, each one coming into view separately and at right angles to 

 the other as the stage of the microscope is revolved. In the limiting 

 case where the particle is lying on a plane parallel to the optic axis 

 the interference figure is practically indeterminate and the position 

 of the grain must be altered. 



Biaxial substances lying on a plane perpendicular to the acute bi- 

 sectrix of the optic axes show the points of emergence of the two optic 

 axes, which points mark the loci of two hyperbolae, extending in 

 opposite directions and momentarily coming together in the form of 

 a cross as the stage is revolved. As the particle is tilted out of its 

 perpendicular position one or other of the points of emergence first 

 leaves the field of the microscope, only one hyperbola remaining in 

 view. As the tilting continues this second point of emergence like- 

 wise leaves the field; but biaxiality is still evidenced by the curved 

 portion of the hyperbola remaining in view. In the limiting case 

 where the particle lies in a plane parallel to the plane of its optic 

 axes, the interference figure is indeterminate and the position of the 

 particle must be altered. In case there is a large acute angle between 

 the two optic axes, both points of emergence are out of the field when 

 the particle is in the perpendicular position. In such a case only one 

 point of emergence can be observed at a time, and tilting is necessary. 



If the interference cross of a uniaxial substance, or the interference 

 figure, preferably revolved into the form of a cross, of a biaxial sub- 

 stance, be observed and a gypsum plate showing red of the first order 

 be inserted, two opposite quadrants of the figures become blue in 

 color and the other two opposite quadrants become yellow. The 

 position of the quadrants colored blue and yellow establishes the 

 optically negative or positive character of the substance under exami- 

 nation; but it is necessary to calibrate each microscope and gypsum 

 plate as to the position of the colored quadrants. This can readily 

 be done once for all by observation of a piece of muscovite. This min- 

 eral, being optically negative, gives the phenomena characteristic of 

 negative substances. For positive substances the phenomena are 

 simpl}^ reversed. In case the interference figures are highly colored, 

 a quartz wedge may conveniently be used in place of the gypsum 

 plate. But here, instead of noting the coloration of the quadrants, 

 one must note the movement of the colored bands into or away from 

 the center of the figure in the different quadrants as the wedge is 

 slowh' inserted. The bands in two opposite quadrants move in to the 

 center and away from the center in the other quadrants. Here, as 

 with the gypsum plate, the microscope must be first calibrated. 

 Muscovite will serve very well for this purpose, a fairly thick piece 

 showing the phenomena. 



