VERTICAL ILLUMINATORS, METALLURGICAL MICROSCOPES 81 



firms, it is evident that the best results will be obtained with 

 illuminators of the prism type, only when the prism can be dis- 

 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. 32 there sometimes 

 remains a slight blurring of half the image. To meet this dif- 

 ficulty, two sliding diaphragms are provided in the Zeiss illumi- 

 nator, 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 exceed- 

 ingly 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 

 demonstrate slip bands in fatigue failure requires that we shall 

 be thoroughly familiar with the optic effects resulting from dif- 

 ferent types of illumination by reflected light. These effects 

 have already been discussed at length on pages 30 and 31, to 

 which the student is referred. 



With ordinary etched metal preparations no special difficulties 



