Dark-field microscopy improves the contrast of specimens that are difficult to visualize under normal illumination. Direct light is blocked, allowing only light that passes obliquely through the specimen to contribute to the image, resulting in a bright sample against a dark background. This technique requires a special condenser to achieve appropriate illumination and is useful for observing a variety of biological samples such as bacteria, spirochetes, hyphae, and arthropods (Supplementary Figs. 1–3). Dark-field microscopes are expensive and not easily accessible.
We present an easy and inexpensive method for converting a bright-field microscope into a dark-field microscope using a coin (Fig. 1). First, light intensity should be increased to the maximum level. Afterwards, the condenser should then be positioned at its maximum height, in close contact with the specimen. A coin measuring between 15mm and 20mm should be placed at the center of the light source. When looking through the microscope, a central dark area with a well-illuminated peripheral field will be observed. Then, the diaphragm should be adjusted by closing it until it matches the edges of the dark circle produced by the coin, thereby eliminating the illuminated field (Fig. 2). Thus, the central area becomes a dark field. Magnification may be increased with small adjustments of the diaphragm.
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