It is important that with darkfield microscopy, the light that illuminates the specimen is a hollow cone of oblique light rather than a solid cone of light. Why is a hollow cone of oblique light needed for darkfield microscopy, and why would a solid cone of light not work for darkfield microscopy?

The oblique illumination is

used by the microscopists to resolve N. Spencrii in the Dark Field Condensers.

Explanation:

It was found that the use of oblique light was required to resolve the striae. Oblique illumination was the first step toward dark field microscopy. In dark field, light

seems to radiate from the specimen while all the rest of the field is black. Resolution is as good as that in bright field while contrast is enhanced.

In dark field microscopy, no

direct light from the condenser enters the objective lens. Only light

that is reflected, refracted or diffracted by the specimen enters the objective. The dark field condenser produces a circle of light. The

light is at an extremely oblique angle to the surface of the slide. This

oblique light comes to a focus on the specimen. It then diverges so

strongly that no direct light enters the objective. This type

of illumination is a hollow cone of light.