Allen B. Dick first described his design for a polarizing microscope in the
Journal of the Royal Microscopical Society (RMS, 1889, pp.432). In this
design the stage remains fixed, while the polarizer and analyzer rotate in
synchronism by gearing with an angle scale readout on the stage. The
microscope was manufactured and signed by J. Swift & Son of 81 Tottenham
Court Rd., London (Swift Catalog, 1891). The instrument became known as the
Dick Model featuring an English foot with a Traviss rolling slide holder.
This petrological microscope differs from the usual example in that it has
a fixed stage instead of the more commonly encountered circular rotating
stage. Instead of rotating the specimen, the two polarizing elements, along
with the eyepiece, are made to revolve together. The crosshairs in the
eyepiece turn, leaving the object stationary. This allows the tiniest
specimen to be maintained in position during the entire rotation,
eliminating the need for delicate centering adjustments. Both the polarizer
and the analyzer are rotated by a set of large gears, with the smaller hand
gear engaged to the large eyepiece assembly just above the fine adjustment.
The rotation is transmitted to the substage by means of a long pinion rod,
which extends down from the hand gear through both the arm and stage of the
microscope.
Just under the stage and part of the condensing arrangement is a large disk
(circle) divided into 360-deg. The disk rotates simultaneously with the
nicol polarizing prisms. A magnifying lens is provided on the front of the
body-tube for easy focus on the graduations of this large disk (visible at
the front of the stage). A low power condensing lens is mounted over the
polarizer, with the higher power element mounted in a slide which can be
slid into place for use with short focal length objectives for conoscopic
work. The specimen is carried by a Will R. Traviss type roller stage
(wheels on the sides). The body-tube features two horizontal slides
(inserts), which pass through the optical path. Each is furnished with a
lens and an open aperture. The lower one is for showing rings round the
optic axis of crystals and the upper one is for exhibiting optic images in
small crystals (conoscopic figures). Both can be adjusted up and down in a
vertical plane. The upper-right side of the stage is divided into mm. in
both directions, for recording the position of an object. The slot in the
eyepiece-tube is for insertion of a micrometer, retardation plate or
quartz-wedge (not present). The fine focus adjustment is a differential
screw motion, with a large milled head divided into 80 parts, each
division being equal to 0.01 mm., making it able to be used for finding the
refractive index of a transparent mineral. A Kleinís quartz plate can be
dropped into the open aperture of the lower slider (on the body-tube); it
can also be used in a holder on the stage (not present).
If the microscope is to be used for non-pol. work, the analyzer is moved
out of position and the substage polarizer swung to the side. The condenser
is furnished with an iris diaphragm and two small stops (both missing). The
accessories include the following: 2 Swift Universal eyepieces, 3
Huyghenian type eyepieces one with crosshairs (adjustable), a triple
nosepiece, 5 Swift objectives all with brass storage cans, and a pinhole
eyepiece cap (used for alignment) all in a fitted mahogany storage case.
The 1891 Swift catalog claims that this microscope was supplied to all the
departments of the Geological Survey throughout the kingdom. The price at
that time was listed, without lenses, at £18.00; complete it was closer to
£32.00.
Some years later, an improved model was introduced by Swift. The stage is
fixed but, if desired, a rotating stage may be attached. The polarizer,
the analyzer (in the cap), and the ocular with its crosshairs may be
rotated together by means of gearing which can be clamped in any position.
The rotation can be read to 5-min. of arc by means of a magnifier just
above the rotating graduated disk at the top of the body tube. Either
nicol (polarizer) prism may be rotated independently or thrown out of the
line of collimation. A second analyzer is fitted in the body tube above
the objectives and there are two Bertrand lenses, one above the other, for
giving either large or small interference (conoscopic) figures. Below the
stage is a turret carrying three different condensers and an iris diaphragm
all of which can be raised and lowered by means of a rack-and-pinion.
Multiple condensers allow the matching of NA to that of the objectives.
The fine focus reads in 1/1000 of a mm. of vertical movement.
