Items to Observe With Polarized Light: An Annotated List

by Richard L. Howey, Wyoming, USA

 

It occurred to me that it might be rather fun and helpful to make up a list of some of the types of objects which are particularly interesting to look at under polarized light and to add a few comments about these objects.  Several articles on setting up your microscope for polarization have already appeared on Micscape, so if you need help regarding technique, you should refer to one or more of those [e.g. click here] or to a good text on basic microtechnique. The list I am presenting here is by no means exhaustive, in fact, it will merely scratch the surface and I will restrict myself to some of my personal favorites.

1) Orange G—This is a biological stain commonly used for contrast and it's a real dazzler under polarized light.  It is of special interest since it produces color shifts throughout the entire range of rotation of the analyzer.  I dissolve this stain in 70% alcohol so that the crystals begin to form relatively quickly and I am then able to watch that incredible process.

2) Rochelle Salts—These are sodium potassium tartrate crystals and are sometimes available at a pharmacy.  These are also of particular interest, since under polarization, some of the crystals show the classic "Maltese Cross".  The one problem with crystallizing Rochelle salts is that they are hydrophilic (water-loving) and they extract moisture from the air, which means that, sometimes, it may take days or even weeks to get them to crystallize.  You can heat them gently on a slide drying table or a hot plate set at a low temperature, but remember that temperature is a significant factor in the types of crystals which form.

3) Copper Sulfate—These are splendid crystals to work with to demonstrate the last point.  If you crystallize them at a series of temperature steps, you will be able to observe an interesting variety of forms of crystallization.

4) Magnesium sulfate—This can be bought in almost any large discount store or supermarket under the name of Epsom salts.  They tend to form either long, needle-shaped crystals or massive clumps.  They are moderately colorful under polarization and would, I suspect, also be interesting subjects for temperature experiments.

5) Mica—Mica is, of course, a mineral which is sometimes  found in large sheets which consist of stacks of very thin layers.  It was also know under the name isinglass and was used as an insulator and also in microscopy as a compensator for polarized light.  The differences in thickness will show up in different colors under polarized light.

6) Miscellaneous Crystals—Often it is very difficult for the amateur to obtain chemicals and reagents that are of interest under polarization.  However, there are many interesting substances that are available as household products which can sometimes produce spectacular results. [CAUTION: Just because these are household products does not mean that they are harmless.  All chemicals must be handled carefully and never mix compounds; you could create a deadly gas.] A drop of household bleach will produce very nice crystals, but they are not birefringent.  However, if you add a compensating filter, such as a 1/4 wavelength or full wavelength filter, you can get fascinating results.  So, if regular polarization doesn't give you any results, try a compensator.  Squares, 2" x 2", of this material, as well as good quality polarizing material, can be purchased in the U.S. from McCrone Microscopes & Accessories.  You can find details on their website: [www.mccrone.com].  A 2" x 2" square of either of the compensators costs $4.00 and since the material is imbedded in plastic, you can cut it to fit the needs of your particular microscope.  The bad news is that McCrone has a minimum order of $40.00, but they do carry a variety of useful micro-tools and round cover glasses (which are difficult to obtain), so it is possible to quickly meet the minimum.  However, if you don't feel comfortable with such an investment, then find some pieces of mica and use them as compensators.

 Back to household products.  A drop of wart remover, allowed to dry on a slide, will give you very pleasing results.  The active ingredient is salicylic acid and it's designed to burn tissue, so it needs to be handled with care.  Liquid toothache medicine is also a substance that produces intriguing crystals.  Photographic developers often contain hydroquinone and these are also well worth investigating.

 Almost every medicine cabinet or first-aid kit contains a small bottle of tincture of iodine—check it out.  In fact, many of the household products that use alcohol as a solvent are likely to produce interesting results.

7) Cotton fibers—The first thing you do is head down to Dixie and learn how to pick cotton and fight off boll weevils, or you can go to your medicine chest if you're in a hurry and remove some fibers from a cotton ball or swab.  These are quite colorful.  Other fibers worth looking at are wool, silk, and linen.  Cotton and linen are plant fibers, but silk and wool are animal fibers.  You will also find it interesting to look at some threads from a spider web.

8) Animal Hairs—If you get really serious about examining some hair, you can take a 30 gallon plastic bag and head for your nearest styling salon (if you go to an ordinary barbershop for males, you'll miss out on all the samples of female hair which has been dyed and tinted and rinsed and bleached and generally tortured).  However, try to pick a salon without some cutesy name like "Hairport" or for balding persons "Hair Today and Gone Tomorrow".  If you live in a large, slightly crazy city (redundant!), find a salon that provides special hairdos for punkers and then you can get samples of fluorescent green, blue, and orange hair.  However, you might wish to reconsider that last suggestion, because if you go into such a district, you might be kidnapped and drugged by denizens from the neighboring tattoo parlor and awake to find "I voted for Dan Quayle" indelibly inscribed on the skin surrounding your humerus, which wouldn't be humorous.

 So, perhaps, the best place to begin is with a small sample of your own hair and you may be surprised by how colorful it is under polarization.  You will notice that at the places where hairs overlap one another, there are color shifts as a consequence of the increased thickness which refracts the light differently than a single strand does.  What little hair is left on my head is dark blond, but the hairs of my beard have a reddish cast along with a substantial number of gray hairs and they are also thicker and rather wiry. I haven't done any careful examinations, but my casual ones suggest that hairs from different parts of the body can show some significant variation under polarized light.

 Another ready source of hairs is your pets.  So, take a blunt pair of scissors, and very carefully, trim just a few hairs from your coati mundi or aardvark, or if you're more conventional, your cat or dog or mouse or horse.  If you have preserved specimens of mammals, these are also a good source of interesting hair samples; for example, I have some preserved bats which I obtained from a biological supply house and their hairs are very interesting.  (The bats themselves come in handy on Halloween.)

 There are professionals who specialize in identifying hairs and fibers, as these can provide significant, even crucial, information for forensic investigations.

9) Plant Hairs—Many plant leaves and stems have minute hairs which are worth examining with polarized light.  Sometimes, with a pair of micro-scissors, you can snip off a few, place them in a drop of distilled water, and have them nicely flattened out for examination.  However, with some plants, it will be necessary to take a very sharp scalpel and slide very thin sections from a leaf or stem, in order to examine the hairs.

10) Owl Pellets—These turn out to be a rich source of hairs from small rodents and occasionally insect parts and small bird feathers.  They can be purchased from biological supply houses and increasingly are being used in high school biology classes to provide students with a cluster of perspectives relating to predation, ecology, and taxonomy and a fair body of literature is available to help one identify the prey from bones and hair in the pellets.  This can be an absorbing project and with a bit of inventiveness and persistence, one can learn a good deal about the bone structure of a variety of small rodents and make correlations with the hairs in the pellets.  Sometimes, as an added bonus, you may discover very tiny bone fragments, which can be ground or sectioned, and such sections can be splendid under polarized light.

11) Dust—No matter how diligent humans are in their cleaning and scrubbing and scouring efforts, the world persists in being an untidy and rather grubby place.  If the compulsive cleaners knew what they were up against, they would never leave their houses and, furthermore, they would drastically alter the interior environment by putting in airlocks, micromesh filters, and sealing all windows.

 My wife and I stopped worrying since we made a major discovery—after four years, the dust doesn't get any noticeably thicker.  Dust balls are interesting accumulations of hair, fibers, soil, pollen, plant parts, flakes of skin, insect fragments, and various and sundry other bits of detritus from the biosphere—a unique, but common, ecological niche.  And, as we might expect, a creature has evolved to take advantage of that niche—the infamous and dreaded dustmite. Various of the bits and pieces in the dust balls are birefringent, but don't expect me to tell you which—I'm not going to reveal all my secrets until I write a book and make millions.

12) Insect Wings and Muscle Tissue—Lacewings, dragonflies, damselflies, even houseflies have transparent or translucent wings which are quite striking with polarized light.  Mosquito larvae can be flattened under a cover glass and the muscle tissue shows up vividly.  I strongly urge every amateur microscopist to make thousands of such preparations every year to help reduce the mosquito population.

13) Fish Scales—The intricate patterns sculpted in fish scales make them ideal objects for microscopic examination and with polarized light you will experience them in a whole new dimension.

14) Vitamins—Ideally preparations should be made from the individual vitamins in pure form.  In practice, these are often expensive and difficult to obtain.  Multiple vitamin tablets contain so many ingredients that, if you get some interesting crystals, there is little hope of identifying them.  Even tablets of a single vitamin, such as, folic acid or vitamin C, which are readily and cheaply available, contain a variety of "inactive ingredients" which will effect the results you get.  For most of us, the best we can do is experiment with vitamins we obtain at our local store.  However, it is still well worth trying some of them out and you will most likely get some results that will make your experimentation worthwhile.

15) Plant Crystals—In many plants, there are structures that are anisotropic and some of these structures are crystals.  In some instances, you can find them simply by squeezing a drop of "juice" from a leaf or stem.  In others, however, it is necessary to cut a thin section and flatten it under a cover glass.  Some of these chemicals which plants produce are defensive armaments and can work not only against insects, but humans as well.  Dieffenbachia contains capsules with tiny needles in them which when crushed release a toxin related to curare.  Rhubarb leaves contain calcium oxalate crystals which have very unpleasant effects on humans, which is why we only eat the red stalks and not the leaves.
 In plants you can find needle-shaped crystals, roughly round or ball-shaped crystals, elongated prisms, and flat leafy crystals.  I'll provide here a list of a few plants worth looking at when crystal hunting: iris, oleander, Dieffenbachia, onion, forget-me-not, Virginian creeper, portulaca, clover, lemma, thistle, mallow gladiolus, rhubarb, fuchsia, primrose, aster, geranium, maple, hibiscus, and spiderwort.

16) Ciliates—Some ciliates contain anisotropic crystals which simply aren't visible by contrast techniques other than polarization.  Fortunately, they occur in our old friend, Paramecium.  They are crystals of calcium oxalate and are probably metabolic waste products which the organism stores.  Under crossed polars, a group of Paramecia can put on a miniature light show for you and then, if you rotate either the polarizer or analyzer just a few degrees, you will be able to see the outline of the Paramecia cells and the birefringent crystals simultaneously.

 Finally, there are starches, thin sections of wood, crystal deposits from your tap water, and all manner of things around the kitchen which will provide hours and hours of delight.  Every amateur microscopist should experiment with polarization and the use of compensators.  These are simple techniques which, with a bit of practice and patience, can produce stunning results.

All comments to the author Comments to the author sent via our contacts page quoting page url plus : ('rhowey','')">Richard Howey are welcomed.
 

Editor's note: The author's March 2002 article in Micscape, 'Crystals and polarization: A few additional thoughts', gives further suggestions for chemicals to view under the microscope.
 
 
 

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