Notes on mica in the Light Train of a Transmitted Polarized Microscope.
Gordon Couger, USA
I was intrigued by Ian Walker's piece in Micscape, A Variable Compensator for Polarized Microscopes ; after reading I started looking around for some mica to work with. I was not very successful finding any that cleaved well until I got a few ounces from a wood stove restorer that sells mica in bulk for making stove windows. I doubt if it is up to the standards used by an electron microscopist but it is sure good enough for anything I want to do, and at $27.00 for a life time supply the price is right. It has a light gray cast like all the mica I have seen in thick sheets he sells. By the time it's cleaved thin enough for use as a compensator it has so little color it is very hard to see. It is as clear enough for any use I have. It has small flaws in polarized light at 100x but they are seldom over one layer thick. It believe it will serve quite well in areas not in the focal plane of the optics as most compensators and wave plates are. These flaws disappear when the mica is cemented to glass. I used Norland 60 UV (sunlight) cure cement. It was still good even though it was almost 4 years old.
I was able to cleave sheets of mica as thin as 0.002 mm; this had 6 sheets of mica in the single sheet. I have been able to get one or two sheet thick layers to start but my old shaky hands have torn the sheet before I got one big enough to use so far. Younger eyes and hands should be able to do better than I. I seemed to have better luck by letting water run on the mica sheet as I was pulling it away. It sure keeps it from reattaching if I drop it.
In Ian Walker's piece, A Variable Compensator for Polarized Microscopes , he used the mica in the configuration of a Berek Compensator [1,2] . In this case the mica is rotated in the light train so the path of the light though the mica is longer. I found it worked well but it also works as a Brace Kohler compensator when the mica is rotated perpendicular to the polarized light. I don't fully understand the nuances of polarized light compensation [1,2] so I included some references in the bibliography.
I cemented the mica to a microscope slide with a cover slip over it before the mica was trimmed away from the slide. Then I sealed the edges with nail polish. The slide and cover slip made a convenient handle to hold the thinnest micas sheets while I measured them with a micrometer.
The next step is to drill a disk from the mica cemented between the slid and cover slip to fit in the slider to go in my microscope above the objective.
All comments to the author Gordon Couger are welcomed.
1. “ Compensators and Retardation Plates ”, by Michael W. Davidson, Mortimer Abramowitz, Polarized Light Microscopy Molecular Expressions .
2. Model 5540 User’s Manual. The Berek Polarization Compensator Patent # 5,245,478
“A Variable Compensator for Polarized Microscopes", Ian Walker, Micscape.
Red Strong on cleaving mica. www.pupman.com/listarchives/1996/may/msg00137.html .
Notes from SPI, a supplier of high quality mica for the electron microscope trade. You know if you need this quality www.2spi.com/catalog/submat/mica-disk.html
Select Stove & Antique Co. A source of acceptable inexpensive mica.
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