ACHIEVING DARKGROUND ILLUMINATION WITH
A MICROSCOPE HAVING NO ABBE CONDENSER
By Chuck Huck, USA
Although most of my microscopes have substage condensers capable of darkground illumination, I have wondered if darkground illumination could be achieved using a simple student-type microscope having a built-in light source, a 0.65 NA condenser built into the stage, and a 6-hole disc diaphragm.
I decided one way to determine if this was possible was to cut out various circles from some heavy black paper and experiment using a microscope slide. I used different diameter circles and found one size that worked fairly well. I used a drop of rubber cement and placed the proper circle on the slide. The slide was placed over the light source (see Fig. 1) and adjusted until darkground illumination was achieved.
The size of the circle worked fairly well using the 4X objective. I have not gone beyond to the 10X objective because I have not experimented with circle diameters beyond the 4X. But it will give you an idea of what can be done with such a microscope. (Fig. 2 shows a worm from a drop of pond water using the 4X objective. The photo was taken with a Kodak digital camera hand-held over the eyepiece.)
Note that the darkground illumination obtained with the student-type microscope is more "pseudo" darkground as compared with a microscope having a focusing condenser. You will have to experiment to determine the correct circle diameter for your own microscope, and for the various objectives. I don't think I would attempt darkground in this manner with anything more than the 10X objective. But this is what our hobby is all about—experimenting.
Comments to the author Chuck Huck are welcomed.
Published in the July 2007 edition of Micscape.
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