Here is an article about the: Machining a Dark field Microscope, that is it possible for a better condenser design that will reduce the time and effort switching between bright field and dark field.  

The experiment uses a modified LOMO Biolam (Multiscope) with more potential in transmitted light than my modified Monolux microscope. The capability includes the phase contrast upgrade for the Multiscope, an aplanatic 1.40 NA condenser, and a ball-bearing rotary stage. Its phase contrast condenser design provided the basis for a prototype to show an answer for quick change between brightfield and darkfield when the darkfield stop must be near the aperture diaphragm of the condenser. The phase contrast condenser allows this rapid switch only for the 10X objective, which needs the phase annulus for the 100X objective to attain darkfield. The open position in the annulus wheel provides brightfield with an aperture diaphragm.  

After some experiments and comparisons from other methods of microscopy, the prototype condenser exhibits that there is an easy design solution that provides rapid switching among the various illumination modes. The imaging tests using Abbe condenser against the aplanatic condenser proves that a good polarized light microscope must have an aplanatic condenser. It also avoids the need to adjust condenser height for high NA brightfield or darkfield after Koehler illumination has been properly established for the 10X objective.  

Oblique microscopy uses sideways (oblique) illumination either by covering part of the light source to give asymmetric lighting, or even an external light source being shone sideways in the sample. It gives the image a 3-D appearance and it can highlight invisible features. A recent technique based on this method is Hoffmann’s modulation contrast. Oblique illumination is an excellent mechanism to achieve the sideband suppression necessary for the image formation process. This system is most frequently found on inverted microscopes for use in cell culture.  

It is capable of resolving very fine specimen detail that is difficult to distinguish using conventional brightfield techniques and ideal for imaging a wide variety of unstained objects such as living cells, crystals, diatoms, and similar transparent or semi-transparent specimens. Attaining settings necessary for oblique illumination can be accomplished by a variety of techniques with a simple transmitted light optical microscope. The easiest methods are to offset a partially closed condenser iris diaphragm, insert an opaque sector stop near the condenser aperture, or de-center the image of the light source. Despite the consequences of the mechanism utilized to establish oblique illumination, the conditions required for image formation remains the same.  

The advantages of oblique microscopy are: it highlights on invisible structures; simplicity of the setup with only basic equipment required; no sample preparation required, allowing the viewing of live cells. But it has the limitation of low contrast of many biological samples and low apparent resolution due the blur of out of focus objects. With these restrictions, potential artificial structures appearing in the image can seriously limit the usefulness of oblique light to examine and quantitatively describe previously unobserved specimen detail.  

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http://www.modernmicroscopy.com/main.asp?article=53