Notes on using a Victorian compressorium on a modern microscope.

  by David Walker, UK.

Keeping live microorganisms still for visual study or photography can be a potential challenge for microscopy hobbyists and various approaches have been recommended over the years. These include:

Controlling liquid film thickness: by fixed depth spacers, or dabs of Vaseline at cover slip corners, or accurate mechanical control with a compressorium.
Adding a 'thickening' agent to water, either proprietary or suitable domestic reagent.
Adding a suitable chemical to water to narcotise organism. (See Richard Howey's article 'Natural Asphxyia')
Using filamentous material to limit movement, either artificial (e.g teased micro fibres) or from the habitat (filamentous algae).

Like many hobbyists I've tried most of the above and adopt one or more depending on the organism type. One approach I haven't tried is the compressorium which is a mechanical device for accurately controlling the thickness of the water film. A splendid illustrated survey of designs both old and more recent has been presented in a Micscape article 'Compressoria' by Mike Dingley.

Compressoria can be expensive so have never been fortunate to try one. My brother Ian though acquired an example as it was included with his splendid Joseph Casartelli (Manchester) Victorian compound microscope, a stand described in his illustrated Micscape article.

For the finest control of microorganisms especially for the serious worker, a suitable compressorium is probably one of the best approaches but for more casual users like myself it's certainly worth deciding if a given design's advantages outweigh its potential disadvantages coupled with its cost. Especially the older designs which may not always ideally suit the modern microscope. The pros and cons may depend on the exact design but for the Victorian design tried, some observations from the author's experiences are offered below.

A beautifully made lacquered brass compressorium supplied as part of a Joseph Casartelli microscope ca. late 1850's. It has the same footprint as a 3x1 inch slide. In this design the upper slip remains parallel to, and moves vertically with respect to, the glass base. The screw is fine enough for precise control of water film thickness and slip support can swing out for preparing a sample.

On my brother's Casartelli stand with single objective nosepiece it works well (and looks fantastic!). Microscopes of this vintage often used objectives as a condenser, as here, which because of its narrow front housing can also focus into the recess.

On a more modern stand like a Zeiss shown above, the raised pillars can prevent ready use of different objectives. A six turret nosepiece, rather than a five shown here, may also encroach on objectives either side, bulky objectives may also be a problem.
Depopulating the nosepiece somewhat can help but the pillar height meant that the objectives had to be set very high to clear these pillars before swinging in a second objective to refocus. This may not be a major inconvenience on many stands with lightly loaded rapid action coarse focus, but on the Zeiss Photomicroscope III with heavily loaded coarse focus I found this tedious.

The lower glass plate is recessed by about 2 mm preventing accurate setting up of Köhler with certain condensers like the Zeiss achromatic-aplanatic phase / darkfield condenser. If the condenser focus is set high to focus in the recess, moving the compressorium with stage controls could scratch the condenser top on edge of black ring.
A long working distance condenser circumvented this but then lost some facilities like darkfield or full immersion work. 

A number of organisms studied also tended not to adopt a relaxed pose when compressed to immobilise e.g stentor just contracted. So preferred giving them more space attached to algae etc on a conventional microscope slide with coverslip where they adopted natural feeding poses (and the algae 'set' the film thickness). This observation may reflect my lack of patience waiting for critters to adopt selected poses under a compressorium but found it easier with other methods.

The precise control of film thickness with a compressorium was certainly more convenient compared with the more hit and miss approach with e.g. Vaseline at cover slip corners (sometimes to the organism's demise).

But overall, for my preferred way of looking at critters, flitting from high to low objectives and trying multiple lighting techniques, I found this design more of a hindrance than a bonus. If looking out for a compressorium design, two key features that may have bearing on its ease of use is height of any fittings and if the bottom plate design can accommodate the focus of the condenser to be used.

Comments to the author David Walker are welcomed.

Footnote: 'NBS' mentioned above was Northern Biological Supplies a UK company formerly run by the late and sadly missed Eric Marson. Other commercial reagents are available.

Acknowledgement: Thank you to my brother Ian Walker for the loan of the compressorium and permission to share an image of the Joseph Casartelli microscope.

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