PETROGRAPHIC MICROSCOPE AS ACCURATE MEASURING DEVICE. 59 
to pass more readily from the quadrant in which the colors rise to that in 
which they fall than is the case with the slower-moving stage. The intro- 
duction of the sensitive plate below the condenser rather than above the 
objective is an advantage, since the optical system is not thereby disturbed 
and the field shifted and rendered less distinct, even to the extent that 
refocussing is necessary. 
(4) The Bertrand lens E, Plate i, Fig. 3, is mounted on a sliding arrange- 
ment which, in connection with the sliding ocular tube, permits of different 
magnifications of the interference figure from 6 to 15 fold. An iris diafram 
is introduced directly below the Bertrand lens and slides up and down 
simultaneously with it. To be of service in this connection this diafram 
should be located precisely in the image plane of the object as formed by 
the objective, for in that plane alone can light be excluded from adjacent 
minerals in the thin section, as was first emphasized by S. Czapski.* To 
accomplish this readily a small lens L, Fig. 1,19 mm. focal length, has been 
introduced in the microscope (Plate i, Fig. 3) above the Bertrand lens; in 
conjunction with the ocular, the lens serves the purpose of bringing to sharp 
focus the image picture in the plane of the diafram in accord with the prin- 
ciple noted above. In place of the small, auxiliary lens L, the writer has 
heretofore used a lens of long focal length and viewed the diafram and 
image directly from the top of the tube. The new arrangement is more 
convenient, however, and obviates the necessity of removing the ocular for 
the purpose of bringing the image to coincide with the plane of the diafram 
by raising the microscope tube. The lens L swings on an axis and can be 
instantly thrown out of the field. A small spring with pointer automati- 
cally indicates the correct position of the lens when thrown into the field. 
From Fig. 32 it appears that, if the diafram were placed in the image 
plane A"B", light could be excluded from any part of the field more effi- 
ciently than with the first arrangement ; the image is more perfect and the 
objective is not disturbed. The objections to this arrangement, however, 
are serious. The Bertrand lens does not come to exactly the same point, 
except in very accurate construction, and the same point in the image is in 
consequence not always brought to focus in the center of the diafram. The 
image can not be examined directly in the position A"B" by the ocular 
alone without the aid of an auxiliary lens. The position A "B" obtains only 
for a fixed position of the Bertrand lens, the objective and the object. As 
Czapski has shown, the best place for the diafram is in the upper focal plane 
of the Bertrand lens, provided the system is telecentric; but the disadvan- 
tages cited above have proved sufficient to discourage the use of the diafram 
in this position. 
For the examination of interference figures directly as they are formed in 
the rear focal plane of the objective, the small cap stop (Plate i, Fig. i) 
with two sets of slides (S\ and S 2 ) at right angles to each other has been 
found useful. This cap fits into the microscope tube and is inserted in 
place of the ocular. By means of the eye-lens a, the image is focussed in 
the plane of the slides and any particle singled out for examination. Because 
of diffraction phenomena the aperture should not be made less than 0.5 mm. 
in diameter, but even with this restriction and with ordinary objectives of 
*Neucs Johrbuch, Beilage Band, 7. 506, 1891. 
