THE OPTICAL ELEMENTS OF THE MICROSCOPE LENS SYSTEM. 
53 
The numerical aperture of the condenser should be high. If it be too high 
for the objective in use and the field is flooded with light, the aperture can 
be decreased and with it the intensity of illumination diminished, either by 
closing the substage diafram (Fig. 37, 6) or by lowering the entire condenser 
(Fig- 37.- <). 
In a thin section illuminated by a narrow beam, the differences in re- 
fractive index of the minerals present are emphasized, the rims of total 
reflection are wide and pronounced, and much of the section appears to 
stand out in relief. The Becke lines are sharply defined. The best out- 
line pictures are obtained by the use of dark-ground illumination, but this 
method of illumination will not be discussed here, as it has not been applied 
to any extent in petrographic microscopic work. In several of the methods 
FIG. 37- 
noted below oblique illumination is employed with low powers and is ob- 
tained most readily by placing the finger between the reflector and the con- 
denser and cutting off part of the light (Fig. 38.). With this arrangement a 
shadow is cast over half the field but the light beams which emerge are all 
confined to one side of the axis. In critical work with high powers this 
method is less satisfactory and is replaced by the methods of dark-ground 
illumination noted above. 
There are still certain factors in the petrographic microscope which tend 
to decrease the quality of the image obtained by the corrected optical 
system and which are rarely taken into account by the practical optician. 
The presence of the analyzer in the path of the rays not only increases the 
optical length of the tube, but seriously disturbs the quality of the image. 
Pronounced distortion results, even when the best type of analyzer is used 
(Thompson prism cemented with thickened linseed oil). In ordinary work, 
where the stage is rotated, this distortion is not detected by the eye, but if 
the analyzer be rotated while the stage remains stationary, the different 
