Resolving everything down to minute detail ...

For a long period in history the light microscope was the only tool which offered visual access to tiny structures and micro-organisms. Hunting minute detail became a competitive sport, fascinating serious amateurs as well as professional scientists. It is not fully clear sometimes whether the detail hunting was performed for its own sake or as a scientific investigation method.

Virtually all 19th century microscopy textbooks explained how to use test   diatoms   in order to check microscope optics and in order to prove that the maximum possible optical resolution was actually available with a given equipment.

In 1886 Ernst Abbe had provided an advanced theory of microscope resolution on the basis of which the construction of so-called apochromatic microscope objectives became possible. Diatoms were used as a touchstone system in order to prove the degree of resolution that had been achieved.

Among the huge amount of diatom species special  test diatoms   like Pleurosigma angulatum  and  Amphipleura pellucida   were used. Scientists tried to resolve the fine structure of the diatom shells "down to lines" (i.e. lines of dots) or "down to dots" (highest resolution).
In particular the diatom Amphipleura pellucida is almost invisible at medium magnification/resolution. Its very faint details become visible only by means of oil immersion objectives and sophisticated substage illumination techniques (filtered light, complicated condensers, raking light iris systems).

Detail of the test diatom  Amphipleura pellucida , resolved "down to dots". Photomicrography from 1891(!) .
Image width ca. 10µm.
Just give it a try and look whether you are able to achieve similar detail resolution with a modern scientific light microscope!
The photograph on the left side was taken by the famous Prof. Henri van Heurck (1838 - 1909), who pulled all the stops available at this time:
he used a Zeiss microscope objective with a numerical aperture of 1.6 (!) - you will not find this extremely high aperture in any of the modern microscope equipment catalogues.
Professor van Heurck used slides and cover glasses made out of flint glass, with a refrative index of 1,72 [ be wise and don't ask you local microscope dealer for this kind of equipment ;-) ]
The embedding medium had a refractive index of  2.4.
Image taken from Carpenter's beautiful textbook, see literature.

Finest detail - symmetry in tardigrades and elsewhere

Centric diatoms tend to be used as splendid demonstration examples for perfect symmetry in nature. But when looking very close you will notice that real life has a tendency towards strange "close-to-perfect-symmetry" structures.

Centric diatom. Diameter ca. 0.2 mm. Very inmpressive: the "range markers" at the outer edge.

Possibly you will happen to know those articles in popular magazines comparing human face left side/right side asymmetry - with a little bit of asymmetry in an otherwise obviously symmetrical system.

Tardigrades are similar in this respect. The fine detail of the back of an armoured tardigrade reveals an axial symmetry following the longitudial body axis. But when looking closer many questions arise: are those armour ornamentations square, pentagonal, hexagonal, circular or possibly a little bit of everything? What is the function of those small knobs within the polygone structures? And there is a little bit of asymmetry within the general symmetry. Does this partial asymmetry arise from subsequent divisions possibly working well but not necessarily perfectly? Does this small animal really need a structure that is so much delicate and so much variable from species to species?
Well, we can buy expensive optical equipment but we tend to end up with a long list of open questions generated by it.

Detail of the armour of an echiniscus tardigrade (animal looking to the left, seen from top).
The detail shows the fine structure of the so-called scapular plate I.
On the left side of the image, barely visible: cirrus A (a long hair) and the clava (a roundish papilla). Oil immersion objective, N.A. 1.30.

Finest details - Engineering and design in the micro world

The oil immersion objective does reveal fine details in tardigrades which allow a little bit of insight into material structures. Though we must be aware of possible optical artifacts at those high magnifications we can assume that e.g. the strength of the tardigrade claws is partly based on "double T" cross sections well known from steel structures like bridges and factory buildings.

Claws at a hind leg of an echiniscus tardigrade. Note the fine structure of the left and right claw resembling a constructional element with "double T" cross section.
Oil immersion objective, N.A. 1.30.

Dentate collar and claws of a hind leg of an echiniscus waterbear, as seen from the side.
Oil immersion objective, N.A. 1.30.

O.K., possibly a little bit shorter than a model's leg but still elegant: fine structure of the dentate collar at the hind leg of an echiniscus tardigrade.
Oil immersion objective, N.A. 1.30.

As far as the dentate collar is conmcerned we had suggested already previously that its main purpose probably is to free the tardigrade from annoying vegetable material when crossing the moss jungles.

For all newbies here at our e-zine: please find an owerview of a complete tardigrade below. So you will not get lost in all those tiny details.

Echiniscus tardigrade, living in the center of the city of Munich, Bavaria, Germany.
Dark field illumination. Length ca. 0.25 mm.

Literature

F. K. Möllring: Mikroskopieren von Anfang an. S. 63 ff [Annex mit einigen Jahreszahlen zur Zeiss-Mikroskopie-Geschichte]. Vermutlich 1979 erschienen.

W. B. Carpenter: The microscope and its revelations. Tafel XI. 7. Auflage, London 1891.

© Text, images and video clips by  Martin Mach  (webmaster@baertierchen.de).
Water Bear web base is a licensed and revised version of the German language monthly magazine  Bärtierchen-Journal . Style and grammar amendments by native speakers are warmly welcomed.

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