8o ELEMENTARY CHEMICAL MICROSCOPY 



placed forward and back with reference to the optic axis of the 

 objective in order that just the proper area of the objective may 

 be covered by the prism. 



When properly adjusted the image of the illuminated prepara- 

 tion should be of uniform intensity throughout and should not 

 have half the field hazy and blurred with a whitish fog. Chang- 

 ing the distances between radiant, collective lens and illuminator 

 and tipping the prism slightly will improve matters, but with 

 illuminators of the type shown in Fig. 34, there sometimes re- 

 mains a slight blurring of half the image. To meet this difficulty, 

 two sliding diaphragms are provided in the Zeiss illuminator, 

 which slip into the slot S, so constructed with two apertures and 

 a central opaque stop as to effectually prevent reflections and 

 passage of rays from the prism in line with the optic axis of the 

 objective. When adjusting the illuminator, first one, then the 

 other, of the two diaphragms should be tried to ascertain which 

 will yield the clearest image, observations being made with each 

 diaphragm inserted to different depths; an exceedingly slight 

 displacement very seriously affects the clearness of the image. 



Interpretation of Appearances with Vertical Illuminators. 

 The investigator is generally dealing with more or less highly 

 polished surfaces and with areas, part of which are polished, part 

 rough and often studded with minute bristling points. Less 

 frequently, as, for example, in the study of material exhibiting 

 fatigue failure, the preparations are polished but are crossed by 

 exceedingly minute cracks or cleavage planes. To ascertain 

 whether the surfaces are polished or mat, whether we have to 

 deal with elevations or with depressions and to enable us to dem- 

 onstrate slip bands in fatigue failure requires that we shall be 

 thoroughly familiar with the optic effects resulting from different 

 types of illumination by reflected light. These effects have al- 

 ready been discussed at length on pages 38 and 39, to which the 

 student is referred. 



With ordinary etched metal preparations no special difficulties 

 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 



