DESCRIPTION OF PLATE i. 
FIG. i . Device for cutting down the field when interference figures from small grains 
are being observed as they form in the objective itself (Lasaulx method). Two sets 
(Si and S t ) of two plates at right angles and sliding in grooves permit the observer to cut 
off the light from any part of the field. Before observing the interference figure, the 
image from the objective is first brought to coincidence with the plane of the stops Si Si, 
by means of the field lens a. 
FIG. 2. Device for obtaining oblique and dark ground illumination, especially with 
high power objectives. Consists of a brass cap which fits above the eyepiece and carries 
a rotating plate c , into which small disks of cover glass have been inserted ; on these, in 
turn, small, thin brass disks of different sizes (0.5 to 3.0 mm.) diameter are cemented and 
serve as central stops when placed in the eye-circle of the ocular. 
FIG. 3. New petrographic microscope constructed forthemost part in the Geophysical 
Laboratory from a large-model Zeiss photographic microscope as base (see also text 
figure i.) T (designating letter accidentally omitted from Plate i, Fig. 3; see Fig. i 
in text), rigid bar connecting two nicols and effecting simultaneous rotation of the 
same ; A , arm connecting upper nicol carriage with T; C, part supporting bar T and 
rotating about stage; B, arm from lower nicol carriage connecting with bar T; by means 
of the screw and crossbar at B, this arm can be instantly released from T and the lower 
nicol either rotated by itself or, after release by a snap-spring, not shown in the figure, 
thrown out of the field altogether. The total angle of simultaneous rotation of both 
nicols by this device is 190. O, new mechanical stage, simple in design and construction 
and fairly dust-proof. HI, stage screws with divisions on head reading to o.oi mm. 
motion of stage plate. Q, sensitive-tint plate inserted above lower nicol, W, and revolv- 
able about microscope axis by means of containing carriage F. M, combination wedge 
above objective; QI a t , fine adjustment screws above objective; U, screw of fine adjust- 
ment device of upper microscope tube; V, iris diafram below Bcrtrand lens, diafram 
opened and closed by turning head V, which is connected with iris diafram by pin and 
ratchet movement; E, pin for insertion of Bertrand lens which moves in an accurately 
fitting carriage, supporting iris diafram V, Bertrand lens E and auxiliary lens L, which 
swings on an arm indicated in text, Fig. i, and is of such focal length that, together with 
the ocular, it forms a small microscope used in focussing the image from the objective 
in the plane of the iris diafram, V. The supporting carriage of V, E, and L, can be 
moved up and down in the microscope tube and the amount of movement read off on 
the adjacent scale, thus obtaining different magnifications (6.5 to 15.2 diameters) of the 
interference figure. G, upper iris diafram directly beneath ocular. 
FIG. 4. Recent model petrographic microscope made by the Bausch & Lomb Optical 
Co., Rochester, New York. In the design of this microscope, which is largely due 
to Dr. H. Kellner, of the Bausch & Lomb Optical Co. (with suggestions by the writer), 
special care has been taken to produce a microscope that is not only convenient but 
also optically and mechanically satisfactory. The optical system is excellent in 
i-vt-ry respect and the mechanical workmanship throughout is of a high order of preci- 
sion. Although simple in design, the plan of the microscope is such that with it 
most of the measurements required in petrographic microscopic work can be made. 
The oculars fit snugly into the draw tube, the Bertrand lens, B, and the upper nicol, N t , 
carriages have wide bearing surfaces and fit accurately. The upper nicol, N tt can be 
rotated through 90, the lower nicol NI through 360. The arm which rotates the upper 
nicol and the degree circle which indicates the amount of rotation are permanently 
attached to the nicol carriage; the connection is in consequence dust proof. Iris dia- 
frams, 7 t and /, arc used both in the draw-tube and in the substagc. The centering 
screws Si, S t , of the objective move parallel with the cross-hairs of the ocular and are satis- 
factory, as is also the steel objective clamp, C. The fine adjustment screw, A , reads to 
o.ooi mm. and is free from lost motion. The dust guard, D, serves to protect the tube 
from dust when the wedges and plates are not in use. Throughout the instrument all 
bearing surfaces have been made wide and with special reference to rigidity and accuracy. 
The addition of a mechanical stage would be an improvement, but would add to the cost. 
