Polarized Light Experiments (Oct, 1934)

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USING A MICROSCOPE FOR Polarized Light Experiments

By H. J. Sexton and O. M. Freeman

WITH apparatus costing less than two dollars to make, the amateur microscopist can now produce and observe polarized light. This opens up a field hitherto limited by the prohibitive cost of the required accessories. It enables the amateur to witness the most beautiful phenomena and conduct the most delicate investigations of which the microscope is capable.

Nowhere in nature are to be found more astonishing and magnificent displays of variegated color effects or more exact delineations than those produced by polarized light in its passage through a simple slide made from a strip of mica, or a thin section of horn or quill. No degree of magnification, however high, will so clearly resolve the limits and boundaries of a specimen composed of layers normally transparent to ordinary light.

Polarized light differs from ordinary light in that the wave vibrations of the beam in all but one plane are retarded or filtered out. The amazing colors and definition visible in the specimen, through which the light wave vibrations in the single plane pass, are produced by interference. An example of such interference in nature is presented by the color rings one sees when lubricating oil is dropped in water.

To construct the underneath part of the apparatus, the following materials are required: three glass photographic plates 9 by 12 cm., or any flat thin glass of similar quality; some thin plate and strip brass; a few small brass machine and wood screws; a short length of threaded brass rod with a nut to fit; a spiral steel or brass spring to surround the threaded brass rod; a short, stiff piece of copper wire; and a small brass hinge. A little black varnish or enamel is required for painting one of the glass plates.

For the construction of that part of the polarizing apparatus which fits within the eyepiece, or ocular, of the microscope, from fifteen to twenty very thin (.18-in.) flat, rectangular 7/8-in. cover slides and a small piece of thin, stiff cardboard are required.

A revolving stage is also needed. This can be conveniently constructed from a 3-in. circular jar cover, some thin plate and strip brass, and a few machine screws. Two pieces of spring brass are needed for holding the slides in place.

Only ordinary ability in the use of tools is necessary, and no tools other than those usually found in the average handy-man’s kit are required to do the work. To construct the underneath part of the apparatus or polarizer, which fits between the microscope horseshoe, the three glass photographic plates are thoroughly cleaned of all film, and each plate is cut into four rectangular plates of equal size by two lines drawn at right angles across its face with a glass cutter. One of these twelve plates is painted on one side with a thick coat of black varnish and allowed to dry. This glass plate with the black back is placed together with eight or ten of the other clean glass plates, and all are mounted together to form a mirror. This mirror is backed with a 1/8-in. thick piece of wood and then attached to a base made of the thin brass plate by means of the small brass hinge. A method of adjusting it at any desired elevation is provided by means of a split threaded brass rod with nut and a spiral spring as shown. Experiment seems to dictate that the angle this mirror makes with the base for best operation is somewhere between 55 and 57 deg., and varies with the number of sheets and the thickness and quality of the glass used for constructing the polarizer.

For that part of the apparatus which fits within the eyepiece, or ocular, and which may be called the analyzer, it is necessary only to cut the piece of thin cardboard to the shape of the template provided in the sketch, and then paint it with the black varnish.

The corners of fifteen or twenty of the very thin cover slides are snipped off with side-cutter pliers. Unscrew the upper lens from a 4X eyepiece, or any eyepiece with the same or longer tube, and drop the elliptical piece of cardboard within the tube until the edge of the cardboard touches the screen which is located within the eyepiece and which will have to be pushed down to accommodate the cover slides. Now drop the snipped cover slides on top of the cardboard. The periphery of the cardboard ellipse should touch the eyepiece tube, and both cardboard and cover slides should lie snugly together in a slanting position across the tube of the eyepiece. The template shown is designed for a standard No. 4X eyepiece, but any standard eye-piece with the same or longer tube may be used. Screw the lens in place, and the analyzer is complete.

The revolving stage is easily made if the directions given on the sketch are adhered to.

EITHER an ordinary microscope lamp with ground-glass front or daylight is used in the operation of the apparatus. The assembled instrument is set up as shown, and the light is allowed to fall on the polarizer, which replaces the microscope mirror. The microscope is focused in the ordinary way, but it will now be observed when the eyepiece is revolved that there are certain positions in which this revolution causes a dark field. The degree to which the field darkens is a test of the efficiency of the apparatus.

The darker the field, the better the instrument.

The variety of slides which may be prepared for observation in polarized light is very large. For the amateur a great number of crystallizations, minerals, and organic substances are available. Perhaps the best and most spectacular slide to start with is a thin strip of mica, or several very thin strips of this mineral placed one on top of the other. When this slide is placed in the revolving stage, and the eyepiece and stage are then revolved, a beautiful series of color effects will be observed. Each color passes into its complementary color as the eyepiece is revolved through 90 deg. The best results are obtained by removing the top lens of the condenser and using a 4X eyepiece with a 4X or 10X objective. However, the instrument works very well without a condenser system if one is not available.

Another slide that is effective and easily prepared is a small piece of rock asbestos placed on the slide with the grain lengthwise. This gives beautiful parallel rods of delicate color tints. Crystallizations of sugar, copper sulphate, or aspirin make excellent slides. Crystallizations may be prepared in the ordinary way and mounted in Canada balsam.

ORGANIC substances that polarize well arc thin sections of horn, fish scales, feather quills, the lenses of fish eyes, and sections of animal bladder.

With some substances, polarized light gives a series of dark rings and crosses. A little grated raw potato shows this effect very well. The large starch cells of the potato are easily discernible under fairly low power, and in each cell, with the aid of the polarizer, will be observed a dark cross which rotates as the eyepiece is revolved.

Polarized light has numerous applications in the industrial arts. It is used by the optician to detect strains in optical glass and in mounted lenses. The jeweler uses it to detect spurious imitations of the semiprecious stones. The chemist puts it to use in determining the concentration of sugar solutions and the identification of some of the sugar groups. It is used in the textile industry for the identification of the various fibers of wool, silk, cotton, or linen, each of these materials having its characteristic colors when examined in the light.

Perhaps the most interesting use of polarized light is its employment by the biologist in the examination of cell and tissue structure, particularly of muscle tissue. Here advantage is taken of the fact that with the aid of polarized light, materials which are normally transparent to ordinary light can be examined in detail, the details of the layer or fiber structure of the material being clearly defined in color. The light is used, too, extensively by the geologist and metallurgist.

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