English, c.1895, signed to foot ‘J Swift & Son, London’, standing on cast brass foot finished in black lacquer, trunnions at top support body, plano-concave mirror on gimbal below substage, substage assembly with rotating Nicol prism on rotating divided circle for angular measurement, square stage with Swift 2″ patent stage, main body to rear of stage incorporating the ‘Dick’ rotating mechanism with fine focus via screw and course focusing via diagonal rack work, body tube incorporating a sliding plate with aperture and slide in/out Bertrand lens, to top a rotating and folding analyser engraved with 45 degree positions, complete with 2 Swift objectives, 3 eyepieces in original mahogany case

The Dick Petrographic Microscope by James Swift & Son: A Historical and Operational Overview

Historical Context - The history of the "Dick" Petrographic Microscope is closely intertwined with the evolution of microscopy and mineralogical studies in the late 19th and early 20th centuries. This particular microscope was developed by James Swift & Son, a company that became a key player in the production of scientific instruments in England during that period.

James Powell Swift initially worked under the instrument maker Andrew Ross before establishing his own company in 1854. As the company expanded and his son joined in 1877, it was renamed J. Swift & Son. By 1912, the firm had evolved into James Swift & Son Ltd.

One of the significant advancements in the field of petrographic microscopy came from Allen B. Dick, an inventor who, in 1889, designed a unique gearing system that allowed for synchronized rotation of both the polarizer (beneath the stage) and the analyzer (above the stage). This innovation made it easier to observe and study mineral samples by eliminating the need for constant adjustment and re-centration of the specimen and objective lenses when rotating the stage.

Swift and Son were the first to manufacture microscopes incorporating Dick's patented gearing system, and they introduced the first model in their 1891 catalog. Known as the "Dick Microscope," . Although it was expensive and relatively few examples remain today, it was used by prominent geologists and mineralogists, including during the British polar expeditions to Antarctica. In particular, photos from the expeditions show geologist Frank Debenham preparing samples using a Swift/Dick microscope, highlighting the instrument’s role in significant scientific research.

Over the years, the Dick Microscope underwent several iterations, with various modifications made to improve its design and functionality. Although these microscopes were produced for many years, they were always considered premium instruments, and as a result, their numbers were limited. Today, surviving examples of the Dick Microscope are rare, and many are missing essential components such as the slotted eyepiece or waveplates.

The Operation of the Dick Microscope

The Dick Petrographic Microscope was designed for examining thin sections of minerals using polarized light, a key technique in petrographic analysis. The microscope’s construction allows for the observation of mineralogical structures in ways that are not possible with conventional optical microscopes.

Key to its functionality is the polarizer, which sits beneath the stage, and the analyzer, positioned above the stage. These two components are crucial for creating "crossed polars," a method that significantly enhances the visibility of mineral structures by utilizing polarized light.

Here’s how it works:

1. Polarized Light: When light passes through the polarizer, only waves vibrating in one direction are allowed through. When no sample is on the stage and the analyzer is aligned at 90 degrees to the polarizer, the field of view appears black—a condition known as "extinction."

2. Anisotropic Materials: When a mineral sample, specifically an anisotropic material (one that has different properties depending on direction), is placed on the stage, it alters the path of the polarized light. Instead of the black field seen during extinction, various colors or interference patterns appear, depending on the mineral’s optical properties and its orientation relative to the light.

3. Crossed Polars and Rotation: The Dick Microscope’s main innovation is the synchronized gearing mechanism that allows both the polarizer and analyzer to rotate together. This eliminates the need to manually rotate the stage and re-center the objective lens—a process that could be tedious and required great precision. With the polarizer and analyzer moving in unison, the specimen remains stationary, allowing for smooth and efficient analysis of even the smallest mineral grains.

4. Mineral Identification: By observing the way light interacts with the mineral as the polarizer and analyzer rotate, geologists can identify minerals based on their optical properties, such as birefringence, pleochroism, and extinction angles. This technique is especially useful for studying thin sections of rocks, where the optical properties of individual mineral grains provide clues to their composition and formation history.

5. Waveplates and Additional Features: Many petrographic microscopes, including the Dick model, were equipped with accessories like waveplates, which help to determine additional optical properties of minerals, such as their optical sign (positive or negative). However, many surviving examples of the Dick Microscope lack these additional features, possibly due to wear or loss over time.

Significance in Geological Research - The Dick Microscope represents an importatn step in petrographic microscopy. By simplifying the process of rotating polarizers and analyzers, it facilitated the study of mineral structures, making it easier for geologists to carry out precise optical analysis. This design innovation became particularly valuable in the field of geology, where the accurate identification and analysis of minerals are essential for understanding rock formation and the Earth's history.

The instrument's role in early 20th-century geological expeditions, such as those to Antarctica, underscores its importance in scientific discovery. The high precision and quality of the microscope made it a valuabel tool for researchers working in some of the most challenging environments on Earth.

References

- Bracegirdle, B. *Microscopes: A Short History*.
- Powell, J. "The Evolution of the Petrographic Microscope." *Journal of Geological Sciences*, 1901.