58 METHODS OF PETROGRAPHIC-MICROSCOPIC RESEARCH. 
Sommerfeldt* adapted this scheme to the polarizer in the draw-tube and 
emphasized the advantages of rotating the nicols rather than the stage in 
the examination of interference figures. In 1910 the present writer, f 
unaware of Sommerfeldt's paper, described practically the same method for 
rotating the nicols, giving particular attention in the construction to pre- 
cision. Recently Souza BrandaoJ has adapted the same device and appar- 
ently also without knowledge of the previous work of Dick and others. 
In the arrangement of Plate 2, Fig. i, the nicol in thedraw-tube is rotated 
and not a cap nicol, thus relieving the difficulty occasioned by the cap nicol 
in the eye-point of the ocular, which decreases the field of view and tires 
the eye of the observer; the rigid-bar connection is built moreover of few 
parts and of heavy, rigid material, in order that the adjustment may not 
be disturbed on rotation of the nicols by a bending or looseness of the device. 
The upper nicol can be inserted or withdrawn from the tube at will; also 
rotated by itself, as can also the lower nicol. In Plate i, Fig. 3, the lower 
nicol is not attached to the condenser and can be inserted or withdrawn at 
will. In Plate 2, Fig. i , this is not the case and the design is defective in that 
respect. The angle of rotation of the nicols can be read off either on the 
degree circle just above the upper nicol or on the circle of the stage. This 
arrangement does not suffer from lost motion and is especially useful in 
the examination of fme-grained rock or artificial silicate preparations where 
accurate centering is not easy, especially if the individual grains are 
mounted in a liquid and tend to shift their position with the slightest motion 
of the microscope stage. In the measurement of the optic axial angle by 
means of the cross-grating ocular the simultaneous rotation of the nicols 
is necessary. Though this particular arrangement has been devised several 
different times by different observers without the knowledge of Mr. Dick's 
arrangement, the credit for first suggesting and first using a rigid-bar con- 
nection for the simultaneous rotation of the nicols belongs unquestionably 
to Mr. Dick, to whom petrologists are indebted for many of the improve- 
ments adopted in modern petrographic microscopes. 
(2) For the accurate measurement of the birefringence, the optic axial 
angle and the extinction angle of mineral plates or grains, the upper part 
of the draw-tube has been modified as indicated in Plate 2, Fig. i. A per- 
manent attachment or holder has been added, into which different wedges 
and plates, mounted in metal carriages, can be inserted. A corresponding 
opening is made in the Huygens ocular to receive such plates and has IHVII 
so designed that the upper surface of any particular plate practically coin- 
cides with the plane of the cross-hairs, so that a scale engraved on the 
inserted plate can be viewed together with the cross-hairs and without 
appreciable parallax. The plates and wedges used for this purpose will be 
described in detail later, together with the methods of their application. 
(3) The sensitive-tint plate is introduced at Q (Plate i, Fig. 3) just below 
the condenser. It is supported by the carriage F attached to the lower rim 
of the substage condenser lens support and can be rotated about the axis 
of the microscope. This arrangi-im-nt often facilitates the determination 
of the ellipsoidal axis of a particular section because it allows the observer 
Xcitvrhr. wisscti. Mikrosk.. 21. 181-185. '9"4: W. 356- Jfo. 1905. 
tAmcr. Jour. Sci. (4). 29. 437-476. 1910; sec also C. L*M. Ztschr. Kryst., 47, 377. 1909- 
JZcitschr. Kryt.. 49. 193, 1911. 
