IJ2 METHODS OF PETROGRAPHIC-MICROSCOPIC RESEARCH. 
THE CONDENSER APERTOMETER. 
In 1905 E. Sommerfeldt* suggested that a micrometer scale be attached 
to the lower focal plane of a specially constructed condenser lens system 
consisting of three plano-convex lenses and so calculated that the scale 
appears simultaneously with the interference figure. 
Later, and without knowledge of Sommerfeldt's work, Dr. H. Kellner, of 
the Bausch & Lomb Optical Company, suggested that a micrometer scale 
might be fitted in the lower focal plane of a highly corrected Zeiss apla- 
natic achromatic condenser of numerical aperture i .4. The writer accord- 
ingly had constructed, by the Bausch & Lomb Optical Company, a coordi- 
nate scale, the divisions of which read directly to angles in air (the distance 
of any division on the scale from the center being / . sin E, where / is the 
equivalent focal length (E. F.) of the condenser and E is the angle in air). 
This scale in the condenser is conjugate to the upper focal plane of the objec- 
tive and is viewed therefore simultaneously with the interference figure, 
either as it appears in the objective alone or magnified by the Bertrand 
lens and ocular. Actual experience with this new scale indicates that the 
distortion is practically negligible in the central two-thirds of the field 
and increases only toward the margin, where the measurements at best are 
not very satisfactory. The great advantage of this device lies in the fact 
that the values of E are read off directly without regard to the optical 
system of the draw-tube, with the result that the determination of the 
Mallard constant or the angle equivalent curve is unnecessary and any 
optical system can be used directly. The coordinate divisions on the present 
scale are 5 apart. In applying this method to actual measurements it is 
essential that the object be located in the upper focal plane of the condenser; 
otherwise distortion results and incorrect readings are obtained. Since in 
ordinary work with parallel polarized light the coordinate ruling is not ob- 
served, the scale may remain below the condenser permanently and is ever 
ready for use in optic axial angle measurements or as an apertometer. A 
careful comparison of the readings of the Zeiss-Abbe apertometer with the 
condenser apertometer has shown that the readings of the latter are satis- 
factory for all parts of the field except near the margin. 
After having located the points in the interference figure with the con- 
denser apertometer, the angular values are first reduced by means of the 
sine formula to corresponding angles within the crystal, and then plotted, 
after which the measurements are made by the graphical methods out- 
lined above. 
SUMMARY STATEMENT OF COURSE OF PROCEDURE IN MEASl 'RING THE OPTIC 
AXIAL ANGLE OF A MINERAL PLATE IN CONVERGENT POLARIZED LIGHT. 
In measuring the optic axial angle, when only one optic axis appears in 
the field of the interference figure the following steps are necessary : 
(1) Locate the points A\ and H (Fig. 86), by use of either the double- 
crew micrometer ocular, or the coordinate micrometer ocular, or the Becke 
drawing-table, or the condenser apertometer. 
(2) Reduce the angles thus obtained to corresponding angles within the 
crystal by means of the sine formula sin * = /3 sin r, where i = observed angle 
of incidence, r = angle of refraction, and /3 = average refractive index. 
*Zitchr. wissen. Mikroskop., 23, 356-362. 1905. 
