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 hyperbole, 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 
simply 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 
slowly 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. 
