82 
ELEMENTARY CHEMICAL MICROSCOPY 
arise, for with vertical illuminators the polished surfaces appear 
bright, the irregular or mat surfaces more or less dark. But to 
demonstrate fissures, cleavage planes, depressions, etc., requires 
that the examination with the vertical illuminator be supple¬ 
mented by very oblique illumination and that due account be 
taken of the directions of shadows with respect to the radiant, 
remembering of course that in the image seen in the microscope 
directions are completely reversed. 
Polarized Light with Vertical Illuminators. — A further aid 
in differentiating between the phases present in a given specimen 
is afforded by employing polarized rays for illumination or ana¬ 
lyzing the light rays reflected from the object. The light rays 
reflected from the polished surfaces of sections of anisotropic 
crystals are quite strongly polarized, as has been already stated, 
while the rays reflected from isotropic crystal sections are not 
notably polarized. It is evident that if we pick out a given phase 
and employ a magnification, such that an area of this phase alone 
fills the field, we may, by studying the nature of the light reflected 
therefrom, often obtain information of the greatest value as to 
the nature of the composition of the specimen being studied. 
But. as has already been pointed out (page 32), studies with 
polarized light made upon opaque objects are fraught with almost 
insurmountable difficulties and require exceptional experience in 
order that reliable deductions may be drawn from the obser¬ 
vations made. 
Nachet Vertical Illuminator.^ — This instrument. Fig. 34, con¬ 
sists of a collimator tube C attached to a cell F, which in turn 
slips into the threaded adapter A and is held in place by the 
thumb-screw B. The adapter A carries at its upper end a male 
screw thread of standard pitch, serving to fasten the device into 
the end of the tube T of the microscope, while F is tapped with 
standard thread for the attachment of the objective 00 '. Lying 
in the axis of the tube C is the reflecting prism P, the .surface 
R of which is silvered, and the outer end L ground convex, thus 
serving the purpose of a plano-convex collecting lens. An iris 
diaphragm whose diameter is adjustable by the knob K is 
' Manufactured by A. Nachet et Fils, Paris, France. 
