Adventures in Chemical Optics C. C. KIPLINGER West Liberty State College, West Liberty, West Virginia
I.
N THESE days of priorities and scarcity of optical mstruments one feels compelled wherever possible to place in active service any unused apparatus or parts. To this end the device herewith described has been developed to fulfill the functions of several optical instruments. The materials required are usually at hand and are not harmed when diverted to this particular use. The optical equipment of the instrument includes the two lenses of a rapid rectilinear doublet from a 4 X 5 camera, each having a focal length of 33 cm., a 1-inch eyepiece, '/a-inch eyepiece, and two microscope objectives, 1-inch and '/a-inch, respectively, either in the original tube mounting, or remounted in suitable metal or cardboard tubes. A simple optical bench of wood is constructed by nailing two strips of wood, 1.2 cm. X 2 cm. X 1 meter, to one face of a bar of the same material, 5 cm. X 5 cm. X 1 meter, the strips to serve asguides for the lens tubes. The latter are large cardboard mailing tubes, 6 cm. in diameter and 5 mm. thick, large enough to admit cork rings to hold the camera lenses in place and thus prevent injury to the brass threads of the lens mount. The tubes are held in place on the bench by bolts aud sheet metal strips. Figure 1 shows the general arrangement of the apparatus. Two small right-angle irons are screwed to the bench adjacent to the microscope objective and serve as a stage. A portion of the tubing acting as a light shield can be removed to give space for accessories. The eyepiece of the compound microscope is mounted in an extension tube sliding in the main tube so as to permit the use of different tube lengths. The
FIGURI.:1.-A,
two camera lenses are mounted so that they oppose each other as on the original shutter. Incidentally, single lenses may be used instead of the achromatic combination, but a t the expense of a considerable loss in definition. The adjustable slit of the collimator consists of two metal plates with central apertures, held together by two small brass bolts. These plates clamp and hold in position two metal strips with beveled edges which can be adjusted by hand to give the desired width of slit. A metal collar attached to the rear plate slips over a metal tube inserted in a cork ring in the end of the cardboard tube. The bench is attached to a stand by a right-angle clamp which grasps an extra long bolt holding the lens tube and metal strip in position. The same device is used to support the front of the bench with a short length of iron rod. SCHLIEREN
Topler' described the method of striae for studying the optical inhomogeneities in gases and liquids. Figure 1. shows the arrangement of an improved "schlieren" microscope for the illustration and application of this principle. The source of light is a 500-watt projection lamp and the condenser is a 500-ml. Florence flask filled with distilled water. The light is focused on the slit, which is a t the focus of its lens so as to form a collimating system. Immediately in front of the second lens is a diffraction screen made of 60-mesh copper gauze. It is fastened to the cork ring with a single screw bolt which permits some freedom of movement for lateral adjustment. The vertical strands of the screen must be parallel to the vertical slit. A second screen of the same material is fitted over the objective of the microscope, preferably a 1-inch objective, and the wires of this screen also should be parallel to the slit. A two-hole ~ b b e rstopper held by a heavy wire support is bolted to one of the stage irons to serve as a holder for a dropping pipet. A glass cell completes the assembly. To use this instrument, the cell is filled with a liquid of known index of refraction and placed on the stage. The microscope is then focused by moving it toward the stage until the field is traversed by many welldefined and colored diffraction bands. The rubber stopper and pipet are adjusted so that the tip of the latter is sharply in focus. The pipet is then filled with the liquid to be tested and placed in position, and a drop is added to the liquid in the cell while the effect is
COMPOUND MICROSCOPE; B, DROPPING P I P ~ T .-
A N D HOLDER ATTACHE" TO RIGHT-ANGLE IRONOF STAGE,D.
LENSAND 60-MBSHSCREEN;L-E, C O I . I . I M A ~ R .
MICROSCOPE
1 Wied. Ann.. m i , 33; also Wooo, "Physical Optics," 3rd ed.. The Macmillan Company, New York. 1934, pp. 93-7.
observed through the microscope. Any difference in the indices of the two liquids will be shown by flow lines or striae, which are meatly emphasized by distorted diffraction bands. This method is very sensitive and is useful for comparing a pure liquid with a samole of the same substance of unknown ~ u r i*t v . Furthermore, the relative densities of the two liquids will be indicated by the direction of flow.
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DIPPRACTOSCOPE
The following assembly is called a diffractoscope and not an interferometer, which i t somewhat resembles, because the present design permits only comparisons, not direct measurements. It is as sensitive as the schlieren microscope and requires no compound microscow. but does not admit of density comparisons. The c&pund microscope is replaced by a tube which carries a cyhdrical lens functioning as an eyepiece. This lens, of lower power than usually prescribed for the Ravlei~hinterferometer, consists of a 75-mm. test tube fiiled-with distilled water. It is inserted in a transverse hole cut in a one-hole rubber stopper which fits the lens tube. The screen, slit, and axis of this lens must be carefully adjusted for parallelism as before. The stage arrangement is the same as in the schlieren microscope, with the exception that the tip of the dropper should be outside the field, which is crossed by a number of bright and somewhat broad diffraction bands if the adjustment is correct. By adding a drop of liquid differing in index from that of the cell liquid, these bands will be distorted and may disappear for the moment, only to reappear when diffusion or mechanical mixing have recreated a uniform field. Normal sodium chloride and potassium chloride solutions show the same index of refraction with the Fisher refractometer. Both of the above methods reveal definite differences between these solutions and are probably sensitive to the fifth decimal place. The three types of glass cells are shown in Figure 5. The first consists of a U-shaped piece cut from a large rubber stoDDer and flanked by two plate class squares waxed with'paraffin, with t h ' o ~ t e r s ~ a c e h e t w e ~the n plates also filled with the wax. Two narrow strips of adhesive tape mapped about the ends of the cell bind it securely. These are also waxed. The cell has a capacity of 3 ml. The second is a small all-glass cell of smaller capacity made according to the suggestions of Wright and Swinglc2 Three plate glass squares 6 mm. thick were cut, and a deep notch chipped and ground a t the middle of an edge of one. They were then fused together in an electric furnace a t 500°C., using the borosilicate cement suggested by these authors. The third cell was made by bending a narrow strip of plate glass into a U-shape and annealing in cotton. The ends of the U were ground flat on a carborundum stone and plate glass sides were cemented on with Canada balsam. A refractometer can be constructed by fitting the 2 "The construction of Tiselius electrophoresis cells," Science 97, 564 (1943).
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