Darkfield microscopy long preceded the use in "biological medicine." While it's been a while since I set it up (i.e., 30 or 40 years, since it isn't used much), you take a regular laboratory microscope and change the light condenser under the stage, and, IIRC, the objective lens. What it then does is let you visualize, bright against a dark background rather than the usual darker (stained) objects against a lighter field, the outlines, and certain internal structures, of microscopic objects. This was most useful for things that were hard to infiltrate with stains for examining them by light transmitted through them, the usual practice. In general clinical medicine, the most common application, beginning to get less common in the 1960s, was verifying the presence of the spirochete of syphilis, Treponema pallidum.
There's nothing inherently wrong with the method for examining appropriate things, but, for most of its applications, better methods are in use -- methods which, in part, don't require modifications to the microscope, or use completely different instruments. Even for tasks such as visualizing T. pallidum, it was easier to coat the organisms with antibodies tagged with a fluorescent dye, illuminate them with ultraviolet, and possibly darken the room somewhat. Sort of "nuke 'em until they glow and shoot 'em in the dark."
Cell widths and counts are now done with automated flow cytometers.
Isn't this fun while I appeal at CZ? Howard C. Berkowitz (talk) 12:04, 15 June 2011 (UTC)