AN IMPROVISED POLARIMETER C. C. KIPLINGER, STATE TEACHERS' COLLEGE, WESTLIBERTY, WESTVIRGINIA That phase of organic chemistry called stereoisomerism is often approached by the teacher of organic chemistry in a small college with a degree of trepidation owing to the lack of equipment necessary for the proper illustration of this branch of the subject. A polarimeter of the usual type finds so little use in the small laboratory that the heavy initial cost is rarely justified by the limited service it renders. The improvised instrument herewith described is amply adequate for demonstration purposes and will permit work being done of a semi-quantitative character. A small ring stand with a flat base is equipped with three small rings, R, R, R, as shown. On each of the lower two rings is placed a glass plate, G, G, to serve as stages, or supports, for the appropriate apparatus. T is a tripod microscope and C a glass cylinder or graduate with as plane a bottom as can be selected from those available. P comprises the polarizer, consisting of two "piles" of glass plates (microscope slides serve well) the lower one coated with a black varnish, lampblack and shellac, or black sealing wax melted on. The first pile lies flat on the base, the second is supported contiguous to the first and a t the optimum angle for polarization by a wedge of cork or wood, K. Bits of adhesive tape help to hold the plates in place. L indicates the course of the light and the double reflection gives the maximum degree of polarization. The writer has found that five plates in each pile are sufficientfor the purpose in hand. One can also get excellent results with two plane black mirrors, if the light source is intense enough. Plates cut from thin negative or plate glass from any source, thoroughly cleaned, are as good as microscope slips. The analyzer, A , consists of a small black mirror mounted with a bit of wai a t the polarizing angle, about 5.5O from the horizontal, in a wedgeshaped recess cut in a small cork. This rests on a larger cork, X, so cut that it rotates easily in the supporting ring. A protractor, D, is attached to this same cork. The top of X and the analyzer should be painted a dull black. Having assembled the instrument as shown in Figure 1, the observer looks down the vertical axis of the instrument by placing the eye a t the point A, thus locating the illuminated field in the black mirror. The eye must be moved as the analyzer is rotated, so as to keep the field in view. The analyzer is adjusted with reference to the polarizer by rotating X until there is the maximum darkness in the field, using as the source of light a 100-watt frosted globe Mazda lamp. This may be intensified by means of a condenser, a 500-cc. flask filled with distilled water. A cleavage piece of selenite, S, thin enough to show high interference colors (this can be split off from a large piece with a pen-knife) is placed beneath T and focused. The selenite is then rotated on its stage until the 2174
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maximum color effects appear between the "crossed" mirrors. The analyzer is now turned until the selenite shows a neutral grey tint. If a solution of cane sugar is now placed in the graduate, the colors will reappear to a certain extent, owing to the fact that the solution has rotated the plane of polarization, and it will be found necessary to turn the analyzer clockwise, that is, to the right, in order to restore the neutral grey. By centering the attention on a section of the selenite plate where red meets blue, the line of demarcation will be found to furnish a sensitive index to a relatively small angular change in rotation. By shading the lamp, wrapping the graduate or cylinder with electricians' tape and protecting the eye from extraneous light, measurements may be made that will correspond to within one degree. This type of polarizing equipment, also described in the literature (I) by the writer may be adapted to the ordinary compound microscope by moving the mirror aside and using the glass plates to reflect the light into the instrument, which must stand in the vertical position. A small cap analyzer as made above is set on top of the eyepiece. Or, if desired, the polarizer may consist of a blackened glass plate laid on the table immediately in front of the microscope and light reflected from it thrown into the axis of the instrument by a second reflection from the plane mirror with which the microscope is provided. The mirror will be at an angle approaching the vertical. A test plate of selenite, or one prepared by allowing a solution of tartaric acid to crystallize on a slide, will aid in achieving the most efficient adjustment of the mirrors. If a drop of a solution of tartaric acid be allowed t o crystallize on a slide and the forming of the crystals be ohsenred between "crossed" mirrors, the phenomenon will be noted as unusually beautiful and impressive.
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If the microscope is equipped with a condenser, interference figures formed by anisotropic crystals may be observed by mounting a Johannsen lens (2). a tiny glass sphere fused on a hair-like thread of glass, the sphere about 0.1 mm. in diameter, mounted as shown in Figure 2 on a split metal cylinder with a lug projecting downward, the cylinder fitting the objective and slidingup or down easily so as to permit the easy focusing of the spherical lens. The latter is held in place on the lug by means of wax, or a bit of adhesive tape. A 16 mm. objective furnishes ample power for this work. A fairly thick piece of mica will show a beautiful interference figure when the adjustment is correct. For good results it will be found necessary not only to focus the microscope, but also the Johannsen lens and the condenser as well. Chamot and Mason's book (3) gives both the theory and technic essential to a successful pursuance of studies in interference figures. In lieu of a standard condenser, the writer has found that an ordinary water lens formed by blowing a thin bulb on a small glass tube, the sphere having a diameter of about 15 mm., cutting the tube to a length of about 2 cm., for a handle, filling with distilled water, and plugging the end, will give moderately good images. I t may be attached to the underside of the stage with adhesive tape. Thin rings of cork are used to vary its distance from the stage opening. A rotating stage graduated in degrees is essenFIGURE 2 tial for serious work in chemical microscopy. The writer believes that it is possible to construct an efficient chemical microscope along the lines suggested and a t a cost much lower than a t present available. The cap mirror analyzer can be used with eyepieces up to 10X without reducing the field to an unreasonable degree, if the mirror is sufficiently small in area. At any rate, many of the most beautiful phenomena of crystals reacting to polarized light may be enjoyed by any one who has a simple microscope and a bit of ingenuity. Literature Cited ( I ) KIP LING^, Sci. Am., 102, 524 (1910). "Manual of Petrographic Methods," 2nd edition, McGraw-Hill ( 2 ) JOHANNSBN, Book Co., Inc., New York City, 1918, p. 454.
(3) C u o T and MASON,"Handbook of Chemical Microscopy," vol. I, John Wiley & Sons. Inc., New York City, 1930, 470 pp.
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