by David Young, Iowa City, USA
The images shown below are all of the same slide; a venom gland from a parasitic wasp. The same specimen was used so as to allow easier comparison of the different forms of illumination tried i.e. brightfield, darkfield, oblique, Rheinberg and oblique Rheinberg. I used my $95 second-hand American Optical 'scope and home-made darkfield/Rheinberg stops. An Olympus D-360L digital camera was used to snap the photos.
To make the oblique Rheinberg effect, I simply off-centered the stop. The effect can be varied by simply adjusting the position of the stop with respect to the optical axis. I've found it simpler to use stick-on black central stops which I pasted onto ordinary microscope slides. The slides were attached to the bottom of the condenser using a bit of Blu-Tak adhesive instead of using the filter holder, which on my AO Spencer student scope is a bit awkward because it's not a swing-out type holder.
It's a simple matter to adjust the position of the stop, just by moving the filter around on the slide by hand. I have even managed to accurately position stops to get Rheinberg and darkfield with the 43x objective and achieved oblique illumination using the 97x oil objective! Do note that my condenser has a flat bottom, so my approach may not work with other types of condensers.
I have even used multiple filters this way, by simply stacking the slides with the stops/filter rings. For example, I have slides with just darkfield stops and others with two/four color filters. Hence it's a lot easier to mix and match filters, since the same darkfield stop can be used with any Rheinberg filter. Note that if using two or more filters in combination, always put the center stop or the filter with the center stop nearest the condenser i.e. 'on the top of the stack' with the colored outer ring directly below it. The diffuser filter, if used, should be at the very bottom of the stack.
Comments to the author David Young are welcomed.
Related articles: Introductory articles on darkfield, Rheinberg and oblique lighting can be found in the Micscape Library-lighting techniques section.
Images by the author.
(Editor's note: To fit in the web page, the author's master images have been resized
and sharpened a little to correct softness introduced by resizing.)
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