A Simple Demonstrating Ultramicroscope HAROLD J. ABRAHAMS Central High School, Philadelphia, Pennsylvania WILLIAM BLITZSTEIN University of Pennsylvania, Philadelphia, Pennsylvania
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E collo~dalstate of matter is of both scientific and industrial importance. Yet most teachers of chemistry are unable to have their students actually see colloidal particles in motion because ultramicroscopes are so expensive. It is hardly necessary to advance an argument in favor of letting students observe the Brownian movement with their own eyes rather than giving them a word picture of this phenomenon. A view of a colloid through an ultramicroscope will produce a stimulating and lasting impression upon students' minds, while a mere description will hardly have this effect. These considerations led us to devise the ultramicroscope herein described. Essentially, an ultramicroscope consists of two parts: a microscope and a dark field illuminator. The microscope in this case is an ordinary instrument with magnification of 150X or greater. The illuminator is simple, compact, and particularly adapted for demonstration purposes.
it consists of only two parts: a lamp and a lens. To produce the bright line most ultramicroscopes illuminate
MICROJCOPE
I FIGURE ER OPTICAL DIAGRAM 08 INSTRUMENT
a slit by an arc light and a condenser. A lens forms a brilliant real image of this slit in the colloid. We have replaced the illuminated slit by its optical equivalent, a bright filament which is in the form of a straight line. (The lamps used are those which are made for the ophthalmic instrument known as the "streak-retinoscope.") A low-power microscope objective is used to OP ULTRAMICROSCOPE FIGURE ~ . S C X E M A TDIAGRAM IC produce an image of the filament in the colloid. ILLUMINATOR The cell or container for the specimen must be transa. Lamp socket parent on a t least three sides (Figure 3). It must allow b. Lamp socket set screw c. Lamp bulb the beam from the illuminator to enter and leave the d. Condenser colloid with a minimum of diffusion and internal reflece. Base of illuminator tion and i t must allow the observer to look down on the f. Set screw for condenser g. Contacts for electricity beam. In our instrument, a semicylindrical slot was h. Cell m. Glass windows As it is the illuminator which embodies most of the interesting features, it will be described in detail (Figure 1). Its function is to produce within the colloid an intense beam of light which comes to a focus which is not a point but a straight line (an effect similar to the Tyndall cone). The optical axis of the viewing microscope should be perpendicular to this line and the instrument should be focused on it (Figure 2). The effect of this type of lighting is to illuminate only those colloidal articles in the verv thin section of the colloid which ar; in focus. Thus kose particles out of focus receive no lizht and do not.flood the field with diffuse illumination. Then each colloidal particle in the beam diffracts the light falling on it into the microscope and appears as a tiny luminous point on an essentially dark field. The optical design of the illuminator is novel in that
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FIGURE3.-PERSPECTIVEVIEW
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Glass windows Intake for filling cell
milled in a block of brass. The ends of the slot were covered by microscope cover glasses sealed to the block
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with paraftin. (Microscope cover glasses were used because they are so thin they have a negligible effect on the optical system.) This made two windows for the beam to enter and leave. A cover glass laid on top of the cell closed in the colloid and eliminated the vibration of the free surface of the liquid. The whole cell was lacquered dull black to reduce diffusion of light. The instrument is small enough to rest on the stage of any ordinary microscope and can be easily moved about, which facilitates adjustment for observation. The electricity for the lamp (2.5 volts) is supplied by a small step-down transformer from the 110-volt AS. mains. Colloidal particles in both liquids and gases may be examined. At the low magnifications used, the visibility of the Brownian movement is slight in l i q u i d s for example, various gold sols' and India ink; however, we have observed it very distinctly and easily a t 150X. As viewed in our instrument, the particles in these gold sols appeared very similar to those pictured in 3 and 4 of Plate 1 in Zsigrnondy's book on coll~ids.~In gases (smoke, etc.) the motion is quite violent and astonishing, offering a striking demonstration of the kinetic
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"Gutbier's Gold," "Gold by Pyrocatechin," "Von Weimam's Gold," "Zsigrnondy's Gold." These were prepared from direc"Colloid chemistry." John Wiley and tions given in HOLMES, Sons, New York City, 1934. a ZSIGMONDY-ALEXANDER, "Colloids and the ultramicroscope," John Wiley and Sons. New York City, 1909.
theory. In both cases it is very easy to see the particles themselves.
