Final answer:
Darker areas on a specimen are illustrated using darkfield microscopy or shading techniques. Darkfield microscopy provides high contrast by blocking central light and capturing diffraction, ideal for live specimens.
Step-by-step explanation:
To illustrate darker areas on a specimen, one would use darkfield microscopy or techniques involving shading. In darkfield microscopy, light is diffracted by the specimen which provides high contrast and resolution, making it particularly useful for viewing live specimens, such as bacteria like the spirochete Treponema pallidum, without the need for stains.
This contrast enhancement is due to the fact that in darkfield illumination, light passing through the center of the specimen is blocked, and only light that is scattered by the edges of the specimen is captured, resulting in a bright image against a dark background.
On the other hand, in the context of illustrations in a book, the darkest and strongest line is used to highlight the most important structures, while shading is utilized to give dimension and shape to the images. This method of using line and color strategically is intended to focus the students' attention on the main learning points without overwhelming them with excessive details.