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Using a Bright Field Light Microscope

Author(s): David R. Caprette, PhD

Find the Target at Low Power

The lowest power objective lens is often called the scanning lens. Scanning lenses are seldom of the highest quality and are not of much use in collecting information. Their purpose is primarily to find a specimen readily and to bring it to the center of the light path and roughly in focus.

In a typical microscope field at 40x (calculated by multiplying the power of the ocular lens by the power of the lens), the field diameter is 5 mm. The advantage of the scanning lens is depth of focus and large viewing area. Although you cannot see much detail, you should be able to find what you are looking for, provided (1) the image is visible in bright field and (2) you know what to look for.

The only concern with finding an object at a very low magnification is that a specimen may not be recognizable. Therefore, it is essential that you know something about your specimen before setting up to view it. Think about the size of the target, how much (or little) contrast it should have in bright field, and how the material is likely to be distributed on a slide. Here are a few suggestions for finding hard-to-locate objects.

Try stopping down the aperture diaphragm (in the condenser) to increase the contrast of the image. Objects will not be well resolved, but the goal at this point is to find them, not to take data. Try focusing on an artifact, such as an air bubble, the edge of a coverslip, or a piece of visible debris. Among the most difficult specimens that are suitable for bright field microscopy are very small Gram negative bacteria. Stained bacteria at low magnification resemble dust on the slide surface. You might use a glass marking tool to make a shallow scratch on the slide surface. (Obviously, you mustn't scratch a prepared slide that is meant to be re-used.) Just as the scratch begins to come into focus, you should be at the level of the specimen, although it still may be hard to find.