This crater is best observed under oblique illumination, as it is otherwise difficult to find.

The image has the appearance of a three-dimensional object under very oblique illumination, causing strong light and dark shadows on the corresponding faces.

If the illumination is vertical, rule 5 is kept; in the case of oblique illumination, it is dropped.

Oblique illumination dispersion staining is the result of refraction and the convex shape of most particles.

With oblique illumination the beam of light illuminating the sample is directed at an oblique angle through the sample.

Oblique Illumination Dispersion Staining (Wright, 1911)

DIC has largely replaced the older oblique illumination (OI) technique, which was available on reflected light microscopes prior to about 1975.

The addition of a light-field illumination system both allowed for additional types of illumination (such as oblique illumination and quasi-dark-field) and correction for optical aberrations.

Though Oblique illumination suffers from the same limitations as bright field microscopy (low contrast of many biological samples; low apparent resolution due to out of focus objects), it may highlight otherwise invisible structures.

Based on drawings made earlier by J. H. Schröter, as well as personal observations and drawings made between 1841 and 1843, he stated that the crater "at the time of oblique illumination cannot at all be seen" (his emphasis), whereas at high illumination, it was visible as a bright spot.