52 METHODS OF PETROGRAPHIC-MICROSCOPIC RESEARCH. 
sliding stops and a scale engraved on the disk, a ray of any desired angle of 
inclination can be had emerging from the glass disk into air or into a refrac- 
tive liquid and thus the aperture of the objective ascertained. 
Objectives may also be tested by use of test objects, as microscopic 
diatoms or mounted bacilli, but for petrographic work the high magnifica- 
tions necessary for the examination of such preparations are not required 
and need not be considered further here. 
THE OCULAR. 
The ocular, which functions primarily as a magnifier, is corrected as such 
for the aberration's of narrow pencils under great inclination to the axis, 
astigmatism, curvature of field, distortion and chromatic differences of mag- 
nification . In the apochromatic systems the compensating oculars are made 
to bear part of the burden in the chromatic aberration correction. Two 
types of oculars are used in petrographic microscopic work: (i) The ordi- 
nary Huygens or negative type, whose field lens intercepts the rays before 
they come to focus in the image (Fig. 30) and produces a reduced image in 
the focal plane of the eye lens, thus increasing the size of the field. This 
type is used in ordinary microscopic work. (2) For micrometer oculars the 
positive or Ramsden ocular (Fig. 32) is preferable because its lower focal 
point, and consequently the micrometer scale, fall outside of the lens com- 
bination. Changes in magnification and micrometer value are therefore 
proportional tochanges in tube length, which is not the case with the Huygens 
eye-piece. The Ramsden ocular in its original form is impracticable and 
the Ramsden eye-pieces commonly used for micrometer eye-pieces are only 
approximations to the original. They are not achromatized as well as the 
Huygens eye-piece, and show color fringes opposite to those exhibited by 
compensating oculars. A practical positive eye-piece can only be made 
satisfactorily achromatic by the use of at least one cemented doublet in 
the combination. 
In both oculars the cross-hairs or micrometer scale are usually so placed 
that the emergent rays are parallel. Under these conditions the normal eye 
is focussed for distant objects and can be used with the least fatigue. The 
drawings, Figs. 30 and 32, represent the object at the distance of normal 
distinct vision, but for practical work with the normal eye these images 
should be infinitely distant if the eye is to experience the least fatigue in 
making long series of observations. 
THE CONDENSER. 
The condenser is designed primarily to furnish a wide cone of incident 
light, its purpose being to send light through the object under the largest 
angle which may be intercepted by the objective. In critical work with 
high powers it is necessary to focus the pencil of light accurately on the 
object point (critical illumination) and the condenser is corrected with this 
end in view. In work with interference figures it is also important that the 
condenser system be aplanatic as well as achromatic. The rays should all 
pass through the same object point if they are to be properly focussed in the 
rear focal plane of the objective, where the interference figure is formed. 
