12 MINERALOGY AND LITHOLOGY. 
of the lower Nicol (as shown by the relationship of its edge or cleavage to the hair lines 
in the ocular), then the light to pass through will be divided into two sets of vibrations, 
one of which will correspond with the axis of greatest elasticity, and the other with the 
axis of least elasticity, their directions in this case being parallel and perpendicular to 
the principal axis. One of these sets of vibration is retarded more than the other; 
and now these two sets of vibrations, on being again reduced to one plane by the Nicol 
above the ocular, are in condition to interfere with one another, which they do, pro- 
ducing color. If we continue the revolution of the table till we have turned it go°, 
then the other axis of elasticity corresponds with the plane of vibration of the light, and 
the field of the microscope becomes again dark; hence such a section, on being revolved 
completely around between crossed Nicol prisms, will be alternately light and dark four 
successive times, and it will be dark each time that an elasticity axis corresponds with 
the plane of vibration of the light, which, in this case, means whenever a crystallo- 
graphic axis corresponds with the plane of vibration of the light in either Nicol. 
The case is different if the section is cut perpendicular to the vertical axis. In 
the ordinary microscope the light reaches the eye in parallel rays; and hence, if 
such a cut is placed in the field of the microscope, the light passes through it parallel 
to the vertical axis. Now, as the lateral axes of tetragonal and hexagonal crystals are 
equal, the crystal is built symmetrically about this axis, and the elasticity in all direc- 
tions in this plane is equal; and hence in this direction a section of a mineral of 
either of these systems acts as an isotrope. It is dark between crossed Nicols, and 
light between parallel Nicols; and revolving it in a horizontal plane between the Nicols 
produces no effect; hence the direction through these crystals parallel to the vertical 
axis is called the optic axis; and, as there is but one direction in these crystals that 
has such peculiarities, these crystals are called uniaxial. 
If a basal section of a mineral of these sections is examined with converging light, 
as is well known, the light no longer passes through the crystal parallel to its optic 
axis, except in the centre of the field of view, but the rays pass through it more and 
more obliquely, according to the distance from the centre. Between crossed Nicols 
this results in the production of a series of colored rings and a black cross traversing it. 
In the Rosenbusch microscope the analyzer is placed above the ocular; and if now we 
take out the ocular and then replace the analyzer, of course the light that reaches the 
eye will be convergent. The field of view will be made small; but, in a basal section 
of an uniaxial crystal, the ring system and the cross will be seen very plainly, and, 
although the picture is small, it is very distinct. By the use of the microscope in this 
way, almost all the effects can be produced that are seen in instruments especially 
adapted for examination with convergent light; and, although Mr. Rosenbusch does 
not refer to it, the clearness and accuracy of the image, as seen in his microscope, 
I consider one of its features. By the use of higher powers, the light is rendered 
more convergent, and the ring system can be seen in quite thin sections, while 
with the lower power the optical properties of the plates that are commonly made for 
optical examination can be very nicely studied. Again: by the use of a quarter undula- 
