INTRODUCTION. 15 
such a plate in converging light is seen in the field; and when a crystal section is laid 
in the field of the microscope in such a position as to disturb the light, the black cross 
and rings are distorted, and do not reach their perfection again till an elasticity axis in 
the crystal corresponds with the plane of vibration of the light. This method of meas- 
uring the angle between crystallographic axes and elasticity axes is accurate, but it 
demands that the crystals have such dimensions as that the black cross can be seen 
upon them. For the study of long, narrow, and small minerals, Mr. Rosenbusch’s 
microscope has a quartz plate cut perpendicularly to the vertical axis, which can be 
inserted directly over the objective. The Nicols being crossed, the field will now be 
brilliantly colored, on account of the revolution of the light by the quartz plate. Any 
desired color can be obtained by revolving the analyzer; but the color selected will be 
modified if a section is introduced in such a way as to disturb the light. Suppose the 
quartz plate to be introduced, and the analyzer to be turned till we obtain a delicate 
violet color; then, if the section of a mineral is introduced into the field of the micro- 
scope, it will appear differently colored at all but at the exact point, whem-one of its 
axes of elasticity corresponds with the plane of vibration of the light, when it will 
be violet. The amount that the section must be turned from this point until the 
crystallographic axis corresponds with one of the hair lines in the ocular, will be the 
angle between the crystallographic and elasticity axes. 
The same holds true in monoclinic crystals cut perpendicularly to an optic axis, that 
was said in regard to orthorhombic crystals, save that these axes bear different rela- 
tionship to the axes of the crystal. As most microscopic observations are made with 
parallel light, the position of these axes is of less consequence in such study. The 
principal point is the position of the axes of elasticity. 
All the principles that have been stated will become plain on consulting PI. 2, Fig. 2, 
which represents two sections of augite from our trap rocks, as they appear in the field 
of the microscope in the positions to be dark between crossed Nicol prisms. Fig. 2a is 
cut parallel to the clinopinnacoid, and is bounded by the edges of the base and ortho- 
pinnacoids. If this section is placed with the vertical axis parallel to the plane of 
vibration of the light, the section will be colored, showing that an axis of elasticity 
does not correspond with the vertical axis,—hence the crystal belongs to an inclined 
system; but, on turning the section about in a horizontal plane 39°, it becomes dark. 
Now, according to Des Cloizeaux, the optic axes of augite lie in the plane of the clino- 
pinnacoid; they make an angle of 59° with one another, and their bissectrix, which is 
the axis of least elasticity in the crystal, makes an angle of 39° with the vertical axis ; 
hence, by revolving this section 39°, we have brought one of the axes of elasticity to 
correspond with the plane of vibration of the lower Nicol, and therefore the light is 
not broken, and the field remains dark. If, now, from this point we revolve the sec- 
tion, it will again be colored; but, when it has been turned 90°, it will again be dark, 
because the optical normal, or the axis of greatest elasticity in a crystal of augite, cor- 
responds with the plane of vibration of the light. If, however, we have a section 
parallel to the orthopinnacoid, this section will contain the orthodiagonal and the 
