56 METHODS OF PETROGRAPHIC-MICROSCOPIC RESEARCH. 
Object points in a plane parallel with the principal plane of the analyzer 
will appear, in consequence, nearer together in the image than correspond- 
ing, equally distant object points in the plane normal to the principal section 
of the analyzer a conclusion which is corroborated by the measurements 
of Table 2 above. 
It would be better, for many reasons, to use only a short cap nicol above 
the ocular, but this arrangement cuts off part of the field and quickly tires 
the eye. There seems to be no satisfactory method at present for over- 
coming this defect and the most convenient place for the analyzer is in the 
draw-tube above the objective. The lengthening of the optical path by 
the upper nicol is often counteracted by the insertion of a weakly magnify- 
ing lens just above the nicol ; this is, however, ordinarily a defect, since it 
only compensates approximately and does not restore the quality of the 
optical image in proportion ; unless centered carefully it causes the field to 
shift and this is a far more disturbing feature than a slight change in focus, 
which is immediately and almost unconsciously adjusted for by means of 
the line focussing screw. The same defects appear on the insertion of a 
quartz wedge or plate just below the analyzer, but these can be obviated by 
inserting the wedge or plate below the substage condenser lens and just 
above the polarizer, as is the case in some English microscopes. 
THE NICOL PRISMS. 
The Nicol prisms of petrographic microscopes are selected with reference 
to their optical quality and their size. The optical quality of a prism 
depends on several factors: (i) the character of the materials of which it 
is made; (2) its design; (3) the accuracy of its construction. The Iceland 
spar used in the construction should be clear and free from all imperfections. 
Many designs have been suggested for Nicol prisms, and if cost be not con- 
sidered that design is best which polarizes the transmitted light most per- 
fectly and gives uniformly illuminated field with minimum loss of light and 
whose angular aperture is the largest. Plane polarized light-waves emerg- 
ing from inclined surfaces (even of isotropic substances) suffer a slight rota- 
tion of their plane of polarization, and for this reason a Nicol prism with 
oblique terminal faces polarizes the transmitted waves less perfectly than 
the square-end Thompson or Ahrens or Glan type of prism. For accurate 
work the square-end type of prism (as the Glan or Thompson) is therefore 
superior to the Nicol prism ordinarily furnished in microscopes. The width 
of the polarizer should be such that it does not act as a diafram and decrease 
the aperture of the condenser. In many of the best modern microscopes 
the polarizer is too narrow and cuts down the available aperture of the con- 
denser to such an extent that in the observation of interfere mv figures only 
a narrow field is available. The disturbing diafram effect of the polarizing 
prism can often be counteracted by lowering the condenser lens, but only 
to a certain extent. If a condenser system of large aperture be used it is 
essential that the width of the polarizer be such that it does not limit too 
seriously the aperture of the condenser. The analyzer should also be of the 
square-end type and of sufficient size so that it does not cut out the rela- 
tively narrow beam of light from the objective; it should not be too large, 
however, as the effects of distortion due to its action increase with its length. 
