128 
BULLETIN OF THE LABORATORIES 
a condensing lense for making the rays convergent. The light is 
polarized in one plane by the polarizer and (the condenser being re- 
moved) passes through the section placed over the aperture in the stage. 
The resulting image is now magnified by the microscope in. the usual 
way. Above the ocular is placed the analyser, consisting of a 
Nicol’s prism set in a revolving cylinder with a graduated limb. This 
prism serves to cut off all the rays polarized in one plane by the lower 
Nicol when its axis is at right angles to that prism. If, however, the 
interposed mineral section is double refractive and rotates the plane of 
polarization of the light, this fact is indicated by the fact that the field 
does not appear dark when the Nicols are at right angles, but at some 
other angle which enables us to ascertain the amount of rotation pro- 
duced by the mineral in question. A quartz plate (zz) is inserted 
above the objective and serves to discover the slightest double refrac- 
tion. An artificial calcite twin, known as the Calderon plate, is in- 
serted in one of the oculars. If the mineral examined be not isotro- 
pous the two parts will be unequally dark, thus enabling us to distin- 
guish the optical characters by a most sensitive test. A plate of cal- 
cite, set in a cork ring, is also used between the ocular and analyzer. 
The interference figure produced by the calcite now is superposed upon 
the mineral section and may or may not be distorted by the action of 
the latter, affording another criterion by which to determine the min- 
erals. 
B. Micro-chemical Methods. 
In the application of chemistry beneath the microscope, tests of 
prime importance are derived from the solubility of the various parts 
of a section and the forms of crystals formed from an evaporated pre- 
cipitate after a reaction is accomplished. 
In order that the tests, may be applied to but a single crystal of a 
section, it is necessary to perforate the cover glass with a minute open- 
ing, thus preventing the uncertainty otherwise unavoidable. An or- 
dinary cover glass is coated with wax and a minute perforation is made 
with a needle, exposing the glass, which is then subjected to the action 
of hydrofluoric acid until the glass is eaten through by a perforation 
less than a millimeter in diameter. The section is then covered and 
the opening is brought directly above the crystal to be studied. The 
balsam is removed by alcohol and the surface of the crystal is thus ex- 
posed to the action. In the case of reagents giving off corrosive 
