Contrast

by Mike Samworth

Contrast is very important in microscopy if detail is to be seen in the subject. Some would argue that it is as equally important as resolution. Contrast in the specimen can be achieved in a number of ways. Some are treatments of the subject itself, such as staining, others are optical methods used during the use of the instrument. To illustrate these differing methods two photomicrographs are shown.

This first photomicrograph shows the desmid Closterium, one that many will be familiar with. To increase contrast dark-field illumination has been employed. Though in this case it is probably done for purely artistic reasons, this type of illumination can make more visible structures that are very difficult to make out otherwise. A useful analogy to use is that of not being able to see small dust particles in a well-lit room, but them being easily visible when lit by a shaft of sunlight coming through a window into a darkened room.

Closterium.

The second method of contrast I have chosen to illustrate is of the specimen treatment type. In this case the specimen is that well-known diatom Pleurosigma angulatum. This diatom is a much-used test object for checking the resolution of objectives. Using a x40 objective the punctae should be resolved as dots, not lines, if the microscope is set up properly. Unfortunately, the frustule is so pale that it is hardly visible, even when mounted in a substance of high refractive index. In the photomicrograph shown, the diatoms visibility has been enhanced by being coated in aluminium, a technique perfected by the late Horace Dall. It is worth noting that although the dots are visible in the original transparency, at the sort of resolution achieved through scanning and subsequent display on your computer screen, they are no longer so!

Pleurosigma angulatum, aluminium coated.

Both photomicrographs by Mike Samworth.

If any reader wishes to ask about any of the above, or to comment, please do get in touch by contacting me Mike Samworth

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