Final answer:
Retroillumination is the illumination technique where the observed feature is viewed in the direct pathway of reflected light. This technique provides enhanced contrast and visualization of certain specimens.
Step-by-step explanation:
The illumination technique described in the question is retroillumination. Retroillumination is a technique used in microscopy where the observed feature is viewed in the direct pathway of reflected light. This technique allows for enhanced contrast and visualization of certain kinds of specimens. In the provided reference, Figure 26.20(d) shows an example of high magnification illumination with reflected light, which is retroillumination.