Nachet Grand Modèle No. 1 - Perfectionné

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The A. NACHET Era: The business operated under several variations of its name. Camille Sébastien Nachet founded the company, followed by the partnership Nachet et Fils. However, when his son Alfred took sole control of the firm after Camille's death, he changed the engraving exclusively to A. NACHET. This specific naming convention was used roughly between 1880 and 1890.

The engraved address, 17, rue Saint-Séverin, Paris, was the location  the workshop moved to in late 1861 and operated out of for decades.

By 1892, Alfred's own son Albert joined the business, and the signature permanently reverted back to Nachet et Fils.

The microscope is equipped with an uncommon internal prism lighting device signed "Nachet à Paris", in its original box signed "Nachet et fils, 17 rue St Sévrin Paris".

The Nachet Vertical Illuminator, also called Nachet prism vertical illuminator or Nachet incident-light / epi-illuminator for opaque objects is a specialized 19th-century accessory manufactured by Maison Nachet & Fils (A. Nachet et Fils), Paris, at their well-documented address 17 Rue St. Séverin. The device itself is signed “NACHET A PARIS”, and the original fitted case bears the full gold-stamped maker’s mark “NACHET & FILS / 17 RUE ST SEVERIN / PARIS” on the red velvet lining.

Key components and light path (matching the classic description in period sources such as Elementary Chemical Microscopy):

• Collimator tube (C): The long brass tube with a convex collecting lens (L) at the outer end. Light from an external source enters here.

• Prism cell (F): Contains a small right-angled reflecting prism (P) whose hypotenuse or relevant face (R) is silvered. The prism is positioned as close as possible to the back lens of the objective — a deliberate design feature that minimizes internal reflections, haze, and loss of contrast.

• Adapter (A): Threads into the lower end of the microscope body tube. The objective then screws directly into the illuminator’s lower threaded mount.

• Iris diaphragm: Adjustable via the side knob (K); can often be shifted eccentrically to optimize the point where light strikes the prism for different objectives/powers.

How it works:

1. External light (lamp, mirror-directed daylight, or modern LED/fiber-optic source) enters the collimator and is concentrated by the convex lens.

2. The silvered prism reflects the beam ~90° downward along the optical axis.

3. The objective now functions as a condenser, focusing a small, intense spot of light onto the specimen surface.

4. Light reflected or scattered from the opaque surface re-enters the objective and travels upward to form the image (most rays pass beside the small prism, which obstructs only part of the aperture).

5. Result: Dramatically brighter, higher-contrast views of surface details, textures, grain structure, inclusions, etch patterns, or bireflectance on gems/minerals compared with oblique or ambient lighting.

Advantages (period and practical):

• Excellent for moderate magnifications on opaque objects.

• Prism very close to objective → superior image quality with less flare/haze than many contemporary designs.

• Controllable intensity and effective aperture via iris.

• Transforms a transmitted-light polarizing microscope into a versatile instrument for reflected-light work.

The specialized optical prism accessory mounted at the top of the microscope, signed “NACHET A PARIS”, was designed as a complementary fitting for Nachet compound or polarizing microscopes of the mid-to-late 19th century. It features a distinctive black-painted (or ebonized) rectangular/prism block with a circular aperture, an angled brass shield or deflector plate secured by screws, precision mounting threads/collar, and integrated optical elements (visible lens/prism face in the side view). The construction uses blackened components to minimize stray light, with brass fittings for durability and adjustment. This version of the camera lucida tilts and swivels, and also offers a deployable filter.

The Nachet top-mount camera lucida is a 19th-century optical drawing aid. It relies on a split-pupil or beam-splitting prism to superimpose a specimen’s magnified image onto a drawing surface. This allows a microscopist to trace the object exactly while keeping their eyes focused on the paper.

Key Design Features

Beam-Splitting Prism: Nachet’s design, famously improved by Dr. Govi around 1882, featured a larger prism with a thin layer of semi-transparent silver or gold. This allowed the observer to view the microscope's visual field and the sketching paper simultaneously with high clarity.

Top-Mount Configuration: The attachment clamps directly to the top of the microscope eyepiece. A laterally extending segment with a mirrored prism projects outward, casting the reflection of the drawing surface into the prism's beam splitter.

Split-Pupil Viewing: To use it effectively, the observer aligns their eye so that exactly half of the pupil is positioned over the prism and the other half observes the paper directly.

This accessory functions perfectly with Nachet’s polarizing microscope system such as this Large Model No. 1, where it supports advanced mineralogical, petrographic, or chemical observations by improving visibility of birefringent structures, inclusions, or textures under varied illumination alongside the vertical illuminator.

The Grand Model No. 1 originally had a full lens array to choose from, including two oil immersion lens for high magnification.

The 1st & 3rd rack segments display Nachet’s standard dry objectives alongside his most expensive, high-numerical-aperture optical engineering marvels:

• The Homogeneous Immersion Lenses (\(1/15\) and \(1/18\)): The markings "imm homog" mean Immersion Homogène (Homogeneous Immersion). This technique was popularized by Ernst Abbe and Carl Zeiss around 1878–1880 to achieve extreme magnifications without optical distortion.
  • Instead of leaving an air gap, a drop of specialized oil with the exact same refractive index as glass was placed between the slide cover slip and the tiny front glass of the lens.
  • The fractions \(1/15\) and \(1/18\) represent the focal length of the lenses in Parisian inches (lignes). These were used for the absolute limits of 19th-century resolution—such as analyzing geological crystal structures, identifying blood cells, or diagnosing early bacterial pathogens like tuberculosis.

• Objectives 1, 3, 5, and 7: These are traditional low-to-high magnification dry objectives. The sequential numbering shows a flawlessly complete, original stepping progression from a low-power overview lens (No. 1) up to a high-power detail lens (No. 7).

The 2nd & 4th rack segments display Advanced Crystallography Optics

The bottom row reveals that this kit was intentionally optimized for advanced petrographic (rock/mineral) or polarizing analysis:

• Compens. Okular (Compensating Eyepieces): This German engraving format ("Compens. Okular" or Kompensations-Okular) refers to high-end compensating eyepieces. These were specially engineered with over-corrected color dispersion to perfectly counterbalance the chromatic aberrations naturally produced by high-power apochromatic and immersion objectives.

• Compensateur 1/4 (The Quartz Wedge/Wave Plate): The specialized brass tube labeled "Compensateur 1/4" is a quarter-wave plate (\(\lambda/4\) retarder). When dropped down into the optical path, it shifted the phase of light waves by exactly a quarter wavelength. Geologists used this to reveal subtle "interference colors" within mineral thin-sections, allowing them to instantly classify unknown crystal optical axes.

The Substage Condenser Assembly: This is the complex cluster of brass underneath the stage. It includes an adjustable iris diaphragm and a condenser lens system, mounted on a rack-and-pinion track so you can precisely crank it up and down to focus light onto the slide.

The Dual-Sided Mirror: Located at the very bottom, the mirror is fully intact in near perfect condition. One side is flat (plane) and the other is concave to concentrate ambient light up through the condenser.

The Grand Model No. 1 utilizes a substage adjustable elevation swing-arm mirror, in near perfect condition, with an elevation knob on each side.

The gear mechanism is contained inside the box secured by four slotted bolts

This image exhibits the key optical element from the Nachet Eclairage Ultramicroscopique accessory: a specialized dark-field immersion lens/cap with a prominent central stop. This is the heart of the ultramicroscopic illumination system.

Technical Description

• Construction: A brass-mounted glass lens (likely plano-convex or specially figured) with a blackened/opaque central zone (the dark circular area in the center). The outer annular portion is clear, allowing light to pass only through the periphery. The rim is threaded or flanged for secure mounting onto the main condenser ring or as a top cap.
• Central stop: Deliberately opaque to block axial (straight-on) rays. This creates a hollow cone of highly oblique light—the defining feature of dark-field/ultramicroscopic illumination.
• Design purpose: When paired with the main brass condenser ring and adjustment plate, it ensures light strikes the specimen at steep angles (high numerical aperture, often requiring immersion oil contact). Direct rays are blocked, producing a black background; only scattered light from sub-microscopic particles reaches the objective.

Function in Operation

• Light path: Incoming light is shaped by the condenser’s concentric surfaces into a wide beam. The central stop eliminates the middle rays, forming an annular (ring-shaped) hollow cone that converges on the specimen plane at extreme obliquity.
• Immersion use: Oil (or appropriate fluid) is applied between this lens/cap and the underside of the slide for optical continuity and maximum aperture. This is why the note mentions “gouttes” (drops) alongside the matched optical glasses.
• Result: Sub-resolution particles (colloids, nanoparticles) scatter light sideways into the objective, appearing as bright points or halos. The field remains dark, dramatically increasing contrast.

This element works in tandem with the brass ring (concentric reflector) and black mounting plate depicted previously.

Background & Significance

Nachet adapted dark-field principles (building on earlier paraboloid/cardioid designs) for ultramicroscopy shortly after Zsigmondy and Siedentopf’s breakthrough (~1902–1903). This accessory allowed their polarizing and compound microscopes to visualize the invisible world of colloids, a major advance in physical chemistry and early nanotechnology. The complete set (condenser ring, adjustment plate, this lens/cap, glasses, case, and note) is a rare, intact example—most surviving pieces lack these precise optical elements.

•  “2 Verres Sphériques [or Optiques] Egaux”: “2 equal spherical [or optical] glasses/lenses.”

•  “2 Dioptrie”: Refers to two dioptric elements (lenses with specific optical power/focusing strength, measured in diopters).

The note is a quick French-language packing list or inventory slip for the spare matched pair of curved/spherical optical glasses (the two clear circular pieces you shared previously) plus their dioptric characteristics or associated elements. The paper shows age-related wear, creases, and a small pink/red seal or tape remnant, consistent with 19th/early 20th-century Nachet packaging.

Accessory Context

These are the matched spherical lenses used with the “gouttes” (drops) to create a thin, curved specimen chamber for ultramicroscopic observation. The curvature (spherical) improves light collection, immersion contact, and reduces aberrations in the high-oblique dark-field illumination—critical for visualizing sub-microscopic particles as bright scattering points (Tyndall effect) against a dark background.

Function and Role in Ultramicroscopy

These components enable proper specimen preparation for dark-field ultramicroscopy:

• Matched glasses: Sandwiched together with a small drop of liquid (the “gouttes”), they form a thin, uniform film or cell. This minimizes optical distortions and allows high-angle illumination without excessive scattering or absorption.
• Drops: The liquid medium suspends the sub-microscopic particles (colloids, nanoparticles, emulsions) for observation. Common in studies of Brownian motion or colloidal chemistry.
• Brass cap: Seals/protects the condenser optics or creates a stable contact point with the slide for oil immersion (essential for maximum numerical aperture and the steepest light cone).

Operation with These Items

1. Place one optical glass on the stage over the condenser aperture.
2. Add a small drop of specimen liquid (e.g., colloidal suspension) onto the glass.
3. Cover with the second matched glass to form a thin, even layer (avoids air bubbles or uneven thickness).
4. Use the brass cap/cover as needed for protection, oil contact, or additional focusing.
5. Align and illuminate via the main condenser ring (as previously described). Particles appear as bright points against a dark background.

This setup is classic for early ultramicroscopy: clean, thin preparations are critical because even minor imperfections scatter light and ruin the dark field.