A Miniature Variable Compensator.


For the older microscope stands.



By Ian Walker.  UK.




Introduction.

Earlier this year I described a simple home made variable compensator for use with crossed-polars to make measurements with thin rock sections and provide an insight into the workings of commercial Berek style compensators at a tiny fraction of the cost of those from Zeiss, Leica et al and is shown below in Fig 1. below. Using a finely cleaved piece of mica the unit is situated on the microscope base between the light source plus polarizer and substage. Well that's fine I thought to myself I was pleased with the results and the overall design but I wanted to take it one stage further and make it available for older microscopes which use an angled stand and external light source. To make it viable it was going to have to be a light self contained unit with the only likely option of fitting it to the eyepiece tube this demanded a novel approach.

Further information on the parts and materials of the original compensator can be found by following the link at the end of the article.






Fig 1.

Original compensator - a heavy base design for a Zeiss microscope with built in illumination,
flat base and horizontal stage.




Fig 2.

Top view of the original compensator.


The idea of this article is not to give detailed instructions, measurements and materials of the unit I have built, but the concept which can be adapted to whatever microscope you might like to use and materials you have to hand including any of your own improvements you wish to make. Ideally you first choose a favourite low power objective in this case my Swift 1" achromat and set the microscope up for optimum contrast with the condenser iris and then fix the unit in position, you could then use other objectives but you will need to know where you normally set your condenser for the higher or lower power objective since you do not want to be continually removing the compensator from the tube to allow setting of the condenser. It is worth making a prototype for your own microscope to find whether it is feasible for your stand, the main point being sufficient eye relief. However I find both the 5x and 8x eyepieces for my Swift stand work well even though the uppermost part of the compensator stands several mm away from the eyepiece although your eye must be held very close to the compensator.


Why use a variable compensator?

The first thing is, I and am sure many others, would like to emulate to a certain extent those accessories normally reserved for the very few and fortunate folk in laboratories who can use extremely expensive Berek and other variable compensators. Unlike fixed compensators such as the 1/4 or full wave plate the variable compensator can provide a range of interference colours which can be useful in familiarizing oneself with the Michel-Levy chart and see for real the colours that are normally reserved for books and which are nothing short of amusing in the variations of print quality. I have several books with the Michel-Levy [birefringence] charts and they differ enormously; so much so it is difficult to know which to trust. You can also do simple experiments determining the birefringence of a mineral in thin section by canceling out the colour and measuring the figure on the compensator, this is detailed in my last article.




Prototype for a miniature compensator.




Fig 3.

The prototype was made from card cut by a compass cutter with additional
low slung counterweight to improve stability, left to right is approx 7.5cm.


To allow the idea to come to fruition I used a working prototype which could be modified to include improvements as I used it in practice and found any shortcomings. The main problem for the prototype is the likelihood of the unit falling off the eyepiece tube and with the unit only weighing 10 grams but with the measuring disk to one side, the centre of gravity tends to pull it off the tube, so I included a small brass counterweight to compensate which worked well. As with my previous design I used a thin cleaved piece of mica which rotates about its axis to provide different amounts of compensation introducing the various interference colours from lower first to mid order second in distinct bands, the colour bands are much more clearly defined than my first design probably because the mica is so close to the eye. The various parts are held together by glue stick and rubber solution allowing them to be dismantled to a certain extent and re-assembled to include new ideas. Some of the parts are from an old video recorder including the steel spindle running from the mica plate to the adjustment disk and the grey plastic bearings are from transistor insulators. A possible source of mechanical parts for projects like this are discarded 3 1/2" and 5 1/4" floppy drives and CD drives from desktop computers where you will find a number of small parts from the mechanism which may work.






Fig 4.

A side shot showing how it fits to the eyepiece tube, eyepiece removed.



In  Fig 4. you can see how the prototype is built up from layers of thin card glued together to increase strength and stability. The lower disk fitting around the eyepiece tube is a precision fit to accurately centre the unit on the microscope. The cheap polarizing disk forming the microscope analyzer [available from Knight Optical UK] is temporarily glued into place on the upper ring with spots of rubber solution easily removed when necessary. The picture gives an idea of the close tolerances between the various parts and eyepiece tube. The length of plastic tube to the rotating measuring disk is calculated to allow sufficient clearance not to interfere with the face when in close proximity to the unit. A delicate touch is required when adjusting the unit so as not to displace it off the microscope. Incidentally the inverted 'L' shape black plastic holding the grey bearings is a small section cut from the cable retainer found in a very old household 13 amp plug [these normally have the two screws passing through it holding the cable into the body of the plug but sadly the more modern ones are flimsy affairs]. However in the last year or two plugs in the UK have become molded and no useful parts, it worked so well it was used in the final version.




The improved and final design.




Fig 5.

View shows calibration marker on lower ring for aligning unit to eyepiece
tube and additional 630nm compensator in situ. Top ring maybe swung-out.

 

For stability the final design main components are made from perspex which gives the unit much longer life and more stable operation than card unfortunately I have no easy way of cutting accurate larger circles in perspex so I have to do it by drilling a number of small holes neatly around the required hole size snapping it out and finishing by hand as shown at the end of the article in Fig 18. The compensator shares a number of concepts from my previous design including the measurement scale built around a small disc shown on the left hand side of Fig 5. A very rigid plastic tube holds this fixed disk in position with its calibrations allowing the steel spindle to rotate within when moving the free disk far left. The analyzer is fixed uppermost on the unit whilst a sufficiently wide window accepts the additional compensator below it without interfering with the field of view looking at the subject. End stops on the calibration disk prevent the mica rotating more than 90 degrees.

Once the perspex has been cut and filed completely round I applied a thin coat of black gloss spray paint to give a nice finish and allows the various calibrating markers shown in white Letraset decals to stand out. A thin slice of Knight Optical full wave compensator cut to the right dimensions and orientation allows an addition to the variable mica plate plate to allow measurements up to third order increasing its versatility over my previous unit.







Fig 6.

Bottom view of unit showing foam base and additional 630nm compensator.


Fig 6. shows the 2 mm thick high density foam which grips the eyepiece tube flange firmly on my stand but allows slight rotation to allow the unit to be aligned accurately in conjunction with the microscope polarizer to give optimum extinction, this could also be achieved by rotating the eyepiece tube with the unit in position if free movement is available on the stand. The mica plate is held by a small rubber cup with a sharp knife cut to accept the plate and it is not held in place by any adhesive. The additional full wave plate which is a slide fit is held by card rings cut in the correct size and orientation as shown above.






Fig 7.

Side view showing mica plate and slot for additional compensator plate above.


Fig 7. shows the sturdy brass pillar holding the two perspex rings apart this has a long brass screw running through it fixing the two together however a nylon washer allows free movement of the upper ring holding the analyzer and additional compensator to be swung out of position and together with the mica plate in its vertical position allows normal viewing of subjects. This picture also shows clearly the additional compensator slot in the top ring in this case end on.






Fig 8.

Home made polarizer mount with long arm facilitates optimum
 alignment in E-W direction with a known source shown in
Fig 9.



Fig 8. shows the matching Knight Optical polarizer swung out of its normal position on a mounting ring with a long arm allowing rotation in the closed position beneath the condenser to provide optimum extinction with a known polarizing source such as the Zeiss Jena eyepiece unit set up N-S shown in Fig 9. Once we know that we have the polarizer accurately set for E-W and analyzer N-S measurements made with the compensator set for 45 degrees to the polarizer following standard petrological techniques. I go through this procedure every time the compensator has been off the microscope to provide optimum results only taking a few seconds to check. The home made mount shown above allows quick removal of the centre polarizer disk to take dark-field stops, rotatable colour Rheinberg rings stackable with the polarizer if desired to give different hues and plain green and blue acetates.







Fig 9.

A known source such as this Zeiss Jena analyzer is used to accurately
orientate the polarizer in
Fig 8





Fig 10.

Overview of the compensator fitted to an old Swift stand, the design
relies on accurate friction fitting of the unit to the eyepiece tube.


Fig 10. shows the home made compensator fitted to the microscope in its normal operating position, the eyepiece tube has been purposely pulled out by about 10mm to show the unit is stable without resting on the flange of the main brass gear tube assembly below but the design was based on the tube being normally closed. One important point which is not clear in Fig 10. is that the hole drilled in the base ring of the compensator must allow the eyepiece to pass through, the compensator is gripping a small knurled flange about 2mm below the eyepiece as seen in Fig 9. which is part of the design of my particular microscope. This is why it is important to look at your own microscope for some time make a prototype and adapt the compensator to fit your own needs. It was common for Victorian and Edwardian brass microscopes to have these larger flanges just below the eyepiece which is a convenient place for the unit to rest and grip.






Fig 11.

Perspex adjustment ring and window, retardation calibrated in nm.


A nice shot of the compensator is shown in Fig 11. here the analyzer can be clearly seen on the uppermost perspex disk held in position by a friction fit card ring. The rotating and measuring disk looks unnaturally large here being a macro shot and distorted perspective. The perspex rotating disk is accurately drilled to be a friction fit on the steel spindle but due to being only 1.5mm thick requires a stabilizing dense foam disk which also doubles as a comfortable and convenient point to rotate the disk.






Fig 12.

Close-up of the mica plate showing the close tolerances
  requiring accurate cutting and measuring of parts.


Fig 12. zooms in on the mica showing the very close working distances between the rotating mica and the top of the eyepiece together with the lower face of the card holding the additional compensator. The 'L' shaped black plastic bearing holder carefully sawn from the cable retainer mentioned earlier can be seen together with the two grey bearings made from transistor insulators the two together giving a smooth damped feel to the rotation of the mica.


Results.

I must stress that my compensator is specifically made for visual observation [your own could be more rigid and improved] since the structure is not strong enough to hold the weight of a digital camera also the camera would need a zoom of 4x with my eyepieces to remove vignetting but for demonstration purposes I have taken some sections from the standard circular field of view using a hand held digital camera sitting lightly on the unit. This gives an idea of the colour range obtainable, Figs 14-15-16. were taken without the additional 630nm fixed compensator. Fig 17. shows a limited view with the 630nm plate in position extending the compensating range to third order. The minimum compensation of my compensator is around 250nm [1st order grey-white on the chart] but this could be adjusted depending on the properties of the mica sample but with the mica in the vertical position most of the original subject can be seen without the effect of the compensator.






Fig 13.

Typical section from a printed birefringence chart for reference below with some of my own rulings.





Fig 14.

With the mica almost horizontal in the light path the lower order interference colours can be seen.

Crop from the camera set at 2.4x optical zoom, now we will increase the angle of mica in
Fig 15.





Fig 15.

A very distinct set of colour bands is seen from 1st order yellow 400nm to 2nd order blue 700nm.

Colours depend on the angle of mica in the light path,  a crop from the camera 2.4x optical zoom.




Fig 16.

Zooming into the colours, 1st order  deep orange 500nm to 2nd order blue 750nm.

Close crop with the camera set to 3x optical zoom showing smoother gradations.




Fig 17.

Using these colours together with a Michel-Levy chart we can do experiments with thin sections...

Crop from camera 3x optical zoom,  +630nm compensator giving 3rd order 1130nm to 1300nm.



And finally, some of the tools and parts.




Fig 18.

Some typical tools and materials that went into building the miniature variable compensators.

Plastic tubing, circular cutter, perspex, small drills, parts from an old video recorder, card and foam etc.


Conclusion.

The compensator can give older biological microscopes including Edwardian and Victorian stands fitted with rotating stages and external light sources the chance to be used as interesting polarizing microscopes with a hint of the exotica normally reserved for the modern dedicated polarizing stands costing more often than not several thousands of pounds. Small parts like the transistor insulators [the same design also used for 1 amp and 3 amp low voltage DC regulators] are most likely still available from large electronic component suppliers online like RS, Farnell, CPC or Maplin in the UK. However with a little ingenuity from your own 'bits-n-bobs' around the house no surplus electronic parts are needed in the main construction such as the bearings for the spindle just a source of material suitable for the compensator plate.

Comments to the author, Ian Walker, are welcomed.

Reference.

A variable compensator for microscopes [My original and larger design written for Micscape earlier in 2006, this includes more detail on mica and experiments with thin rock sections].


Related articles


Notes on mica in the light train of a transmitted polarized microscope - Gordon Couger provides some valuable notes on sourcing and cleaving mica for homemade compensators in his article in the November 2006 issue.

 

 

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