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. 
j_ With ordinary etched metal preparations no special difficulties 
