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. 1 - - 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 OO'. 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 

 1 Manufactured by A. Nachet et Fils, Paris, France. 



