Dark field microscopy is a popular technique to observe transparent specimen. The center of the light source are blocked by non-transparent circle. The light comes from the side to increase the contrast. Dark field microscopy can be easily achieved by placing a coin between the light source and the condenser.
Setting it up: To make the coin working properly, you should center the coin so that the light comes from the peripheral evenly from all direction. Move the abbe condenser up and down so the specimen are not completely in the shadow and not completely in the light either. You should see your specimen with the largest contrast without too much glittering.
Taking photos from Dark Field: The typical setting for the camera are designed for normal light condition. This is too bright for the dark field. I reduced the exposure compensation to -2 when taking photos with my Nikon S200 snapshot camera. The picture below did not use any exposure compensation. The background end up too bright.