Successful polarised microscopy of NWA Sahara (Morocco) meteorite slide using Zeiss IM microscope & Optovar

It has taken me some time to get there but I am here now – success with polarised microscopy through the Zeiss IM microscope!

The following images are of a Chrondrite-rich NWA meteorite from the Sahara desert, Morocco, purchased from SDFossils on May 2018. There is no date on the slide nor meteorite designation number.

It wasn’t obvious exactly which bits were needed but eventually I found that a Zeiss 47 36 68 polarised filter fits into the filter slot below the objective turret – this gives plane polarisation alone. However the purchase of another polarised filter (this time a modern one) designed to fit in the filter slot above the condenser (if you are used to normal upright microscopes remember this is an inverted microscope so turn everything upside down in your imagination and it then makes sense) allows for crossed polarisation. The latter filter can be turned through 90 degrees.

The following images show the Zeiss 47 36 68 filter and where it fits onto the microscope:

The following images show the second polarised filter and where that fits onto the Zeiss IM microscope – both the above filter and this second one are needed to achieve crossed polarisation:

This is also the first time I have used my new Optovar – this is another ebay purchase with optics in excellent condition.

Optovar on Zeiss IM microscope:

The different positions of the Optovar’s ens wheel showing different magnifications offered – these are in addition to that provided by the eyepiece/camera and objective lens:

The Optovar was not originally designed for the Zeiss IM microscope nor for the dual head block I am using to allow me to place a camera below the binocular head. This extra round accessory with the black lever on it is necessary to act as a spacer between dual head block and Optovar so they can fit together, otherwise the lens on top of the Optovar projects too far out and so does that on the bottom of the dual head block and they won’t fit together. The extra accessory with the level is in fact an aperture diaphragm attachment for use with illuminators and I have another one under my illuminator (see diagram above):

The following information about the Optovar comes from

The Optovar magnification changer was originally introduced by Carl Zeiss Oberkochen / West Germany in 1954 as an accessory to the Stand W. This microscope was the first post-war microscope designed by Dr. Walter Kinder at Oberkochen and incorporated a number of major innovations. As such it deserves a separate essay. What I would like to discuss here is the Optovar magnification changer. Still today it is a much valued accessory to any Zeiss microscope of the Standard series and has, in principle, been incorporated in many newer stands since.

The Optovar is an intermediate tube with a magnification changer, it also features an Amici-Bertrand lens (=auxiliary microscope) and an analyser.

The multi-step magnification changer introduces factors of 1x, 1.6x, and 2.5x .Later versions have the factors 1x, 1.25x, 1.6x, and 2x. The Optovar integrated in the larger Universal, Photomicroscope, and Ultraphot offers the factors 1.25x, 1.6x. and 2x. This allows the microscopist to bridge over the magnification gaps between objectives in small steps and saves him from having to change the eyepieces frequently. The optical systems to achieve this are arranged on a rotatable disc and can be switched in as desired.

For instance, with a standard set of objectives 2.5x, 10x, 40x, and 100x and an eyepiece 8x the following magnifications can be obtained:

20 – (25) – 32 – (40) – 50 – (63) – 80 – (100) – 125 – (160)

200 – (250) –320- (400) – 500 – (630) – 800 – (1000) -1250 -(1600) and

2000x (= the DIN series in steps of 1.6x, steps of 1.25x in brackets – figures are rounded off).

The Amici-Bertrand lens as it is officially called, together with the eyepiece, forms what is commonly also known as a phase contrast centering telescope or an auxiliary microscope, and is well known to users of polarizing microscopes. It serves to view the rear focal plane (exit pupil) of the objectives. In other words: it shifts the filament plane into the image plane. In the Optovar, the A-B lens is set in a helical mount that can be controlled by a separate wheel so one can focus up and down to reach the rear focal plane of objectives from 16x to 100x. With it the microscopist can see, focus on, and center the phase rings of his phase-contrast system. He can also check the setting of the condenser aperture diaphragm and the correct centering of the filament of the light source.

It is a most useful tool to inspect the optical system for any misalignment, vignetting, dirt in the objective or air bubbles in the oil immersion. With it one can also detect any cracked or damaged lenses, fungus or delamination in the objective. The main convenience is the elimination of having to remove the eyepiece and to insert the auxiliary telescope each time one wants to check the objective’s aperture or the phase rings.

Lastly, the Optovar includes a swing-out analyser or slot to insert one. The biologist needs only to place a polarizer under the condenser to render his instrument into a simple polarizing microscope to examine crystals or other birefringent material.

All images below with Bresser 5MP Microcam through Zeiss IM microscope, bright field. The advantage of polarised light is that it brings out birefringence in the crystal structure of the rock which can be used to identify the type of mineral in the rock and gives pretty pictures! The advantage of doing this with a biological microscope is the ability to use a range of objectives and imaging accessories that might not normally be available on polarised microscopes – however the disadvantage is that the biological microscope is more likely to suffer from strain in the glass of the instrument and accessories and objectives, which itself will affect the images produced.

Medium resolution images of NWA meteorite:

25x Leitz objective:

Unpolarised light:

Plane polarised light:

Crossed polarised light:

Effect of Optovar on above meteorite thin section showing extra magnification this accessory provides:

x1.0 lens:

x1.25 lens:

x1.6 lens:

x2.0 lens:

High magnification images of NWA meteorite chondrite:

x63 objective + Optovar:

x63 objective, x1.0 Optovar lens = x63 (plus magnification from camera = approx. real magnification of 1758 times – see calibration slide below):

x63 objective, x1.6 Optovar lens = x101 (plus magnification from camera – see below for calibration to convert this into actual magnification). Individual crystals can be seen and it is also obvious that they have a preferred direction:

Calibrating above to give actual magnification – for this purpose I used a calibration slide where each division is 0.01mm.

Calibration slide 0-01mm per division x63 obj x1-6 Optovar 090618:

Therefore across the slide photo there are 7 divisions or 7 x 0.01mm = 0.07mm. On my laptop computer screen the photo appears as 197mm across, meaning that magnification of the on-screen image compared to the original structures on the slide’s actual size = 197/0.07 = 2814 times magnification.

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