Vintage Visits: The Rheinberg Filter Repair Shop

The bits pictured below spilled out onto the table from a small brass container holding several colored glass filters and contrast accessories. The vibrant colorful shards and circular black disc suggested a version of Julius Rheinberg’s differential color illumination filter. The filter components had delaminated from the degraded resin which held all the filter pieces together on a thin glass substrate.

The pieces from our Rheinberg filter appeared to be vintage and professional grade, but what type of Rheinberg differential color illumination filter did we have, and could it be restored?

Starting with the Stops

Julius Rheinberg first introduced differential color illumination during a demonstration of the technique at the Quekett Microscopical Club on December 18, 1896. Below are Rheinberg’s introductory remarks followed by portions of the transcript of his demonstration to the group, which were later published in an 1897 issue of the Quekett Microscopical Club journal.

Rheinberg begins his demonstration by explaining a variety of microscopical central stops he has positioned below the substage condenser of his microscope—19 stops in all. These stops are colored and essential to his new microscopical contrast technique.

“…it consists essentially of a box, or slide carrier fitted under the condenser, in which there are a number of metal slides which can be pulled out or pushed in quite independently…In the apparatus I have here there are 19 stops.”

Rheinberg’s differential illumination is like darkfield contrast, but rather than producing bright microscopical particles against a black background, Rheinberg imparts a single color to the specimen, contrasted against a different colored background. This stark color difference provides an advantage for viewing unstained transparent microscopical structures, which are difficult to see using traditional darkfield microscopy.

During his demonstration, Rheinberg suggests that a more elegant apparatus could be constructed by a skilled instrument maker, suggesting his device could be fitted into any diameter microscope condenser.

Selling Rheinberg

Shortly after Rheinberg’s Quekett demonstration, microscope companies began selling his “multi-coloured discs” as a microscopical accessory. Some of the first companies to offer Rheinberg’s colored discs were W. Watson & Sons, and C. Baker Microscopes & Accessories Company, both located in London, and both listing the filters for sale in the 1900 editions of their catalogs.

The C. Baker description of their multi-colour filters also includes a reference to the latest paper by Rheinberg published in the Transactions of the Royal Microscopical Society, 1900.

Over the coming decades, both C. Baker and W. Watson continued offering Rheinberg differential illumination color filters, selling them in both glass and gelatin varieties. Sometimes the glass filters were simply-cut gelatin sheets cemented between two pieces of glass with Canada balsam. This protected the gelatin filters and made them more transparent.

The above advertisement from C. Baker also lists some of the advantages of using a Rheinberg filter when viewing certain types of microscopical specimens, such as crystals, plant sections, and layered materials.

M is for Microscopy

The gelatin variety Rheinberg filters were made from Wratten M-series filters manufactured by Kodak. Wratten color filters belonging to the M-series were specifically designed to be used for visual work with the microscope. Kodak offered nine different M-Series Wratten filters, which came in two diameters: 35mm and 30mm.

Over the years, the shapes and arrangements of the individual-colored gelatin pieces become more complex, resulting in what was called a sectored Rheinberg filter. There is a nice illustration of sector Rheinberg filters in George Needham’s book The Practical Use of the Microscope (see lower right). Needham also provides a key for each of the M-series color filters that make up the Rheinberg illumination stops.

Based on Needham’s illustration, our vintage Reinberg filter is a six-sector variety. Needham’s filter illustration and the Wratten M-Series filter key provide a nice reference guide to what the original arrangement was for each colored gelatin filter.

Mirror Images

The remaining dried material on the glass substrate left a distinct pattern behind, providing a template of sorts as to how each of the six filter segments of our Rheinberg filter were originally oriented.

As you can see from the side-by-side comparison above, Needham’s illustration is not in the same orientation as the outlined dried resin material on our glass substrate; they are mirror images of one another. To remedy the orientation problem, Needham’s image was simply flipped in the horizontal direction using PowerPoint.

Our thought was that by placing the vintage glass filter substrate over a printed-to-size copy of Needham’s six-sector graphic, we would be able to better visualize the proper order of the six Wratten filter pieces. A legend was also created for each of the filer segments and placed under the Needham illustration, along with the opaque central stop portion of the Rheinberg filter off to the right.

Before the vintage six-sector Rheinberg filter could be reconstructed, the dried resin material had to be removed from the glass substrate and the individual Wratten filter pieces. We suspected the resin material was Canada balsam, based on its color and being a common bonding material used in microscopy.

A small portion of the resin was analyzed by micro-FTIR and was compared to a fresh sample of Canada balsam. The analyzed resin and the fresh Canada balsam had similar IR spectra, but were not a perfect match, perhaps due to degradation of the Rheinberg filter’s resin from age, storage conditions, or processing; such degradation often manifests as broadened and shifted IR peaks.

Fortunately, the old Canada balsam was brittle and easily removed from the Wratten filter pieces by gently applying pressure to each filter using a wood toothpick. A razor was used to scrape off the Canada balsam adhering to the glass substrate.

With a better understanding of the history behind our six-sectored Rheinberg filter and how it was made, the individual filter pieces were placed on a glass substrate and reassembled in place, dry.

Notice how different the individual Wratten filters and central stop appear under reflected light versus transmitted light conditions.

Restoration

To start the restoration process of our vintage six-sector Rheinberg filter, a new one-ounce bottle of modern Canada balsam was purchased from Wood Finishing Enterprises. The new Canada balsam was used to bond the individual Wratten M-Series filter pieces back onto the original glass substrate.

Our vintage glass substrate, which holds the filter pieces in place, is simply a thin flat piece of glass measuring 0.58 millimeters in thickness with a diameter of 34 millimeters. The new Canada balsam was brushed onto the glass substrate using a small paint brush. The Wratten filters were individually placed onto the glass substrate according to Needham’s six-sector Rheinberg filter illustration, which was placed beneath our glass substrate.

After the Wratten filter pieces had been assembled back to their original positions, the central darkfield stop was added to complete the Rheinberg six-sector filter.

What about the black material around the perimeter of the original filter?

Our vintage Rheinberg filter has a ring of black material painted around its perimeter. An IR analysis indicated that the material is similar to a copal gum-based varnish. Black varnishes were typically used to seal microscope slide preparations as well as optical accessories like our Rheinberg filter.

We opted not to apply the copal varnish as a final step because it appeared to have no lasting effect on the preservation of the Rheinberg filter. In the end, the real failure of the filter was due to the degradation of the Canada balsam. You’ll notice in the middle image that we left some of the copal varnish that had made its way to the sides and underneath the glass substrate. We kept the residual copal black varnish to be used as a guide to keep the Wratten filter components centered during reassembly.

In addition to the filter we were restoring, we referred to a second incomplete 27mm diameter Rheinberg filter from the Brooks Collection, which had also degraded. You’ll notice that this Rheinberg filter does not have any black varnish painted along the perimeter.

The apparent lack of structural support provided by a coating of black varnish and the lack of black varnish on the 27mm filter were the primary reasons for not applying a new layer of copal black varnish to our 33mm filter. Of course, the copal black varnish could be easily applied later if desired.

Some Sweet Images

Now it was time to put our vintage six-sector Rheinberg filter to use. But trying to use the restored Rheinberg filter with a modern Olympus BX51 condenser proved to be challenging. There was no easy way to hold the filter in place, and the size of the 9mm opaque central stop was too large to get the desired effect with our Olympus optics. Instead, the Rheinberg filter was placed inside the transmitted light base of an Olympus SZX10 stereomicroscope.

Based on Needham’s notes about four and six-sector Rheinberg filters being especially useful in revealing crystal faces: “The four and six sector stops reveal the faces of chemical and mineral crystals”, a prepared slide of sucrose was placed several inches above the transmitted light port on a mechanical stage.

Indeed, the sucrose sample was an ideal specimen for our six-sectored Rheinberg filter and highlighted the well-formed crystal faces with various colors from the filter.

Our restored six-sector Rheinberg filter should be good for another century or so. And if you happen to find a degraded Rheinberg filter, repairing it will be a colorful experience.