Final answer:
The illumination technique described in the question is known as indirect illumination, which, along with techniques like dark-field and bright-field microscopy, is used to enhance the contrast and details of specimens at higher magnifications.
Step-by-step explanation:
The illumination technique described where the observer focuses the microscope sharply on an area immediately adjacent to the illumination position is known as indirect illumination. This technique is used in microscopy to observe specimens at higher magnifications and requires intense lighting due to the dilution of light as images are magnified.
In dark-field illumination, a condenser system is used to block light passing through the center of the specimen while allowing the light that passes through the periphery of the beam to be diffracted by the specimen. This leads to an enhanced contrast, especially for live, unfixed, and unstained specimens, allowing for better visualization of details.
On the other hand, bright-field microscopy is the most common form of light microscopy, where the specimen is illuminated from below and contrast is achieved through the difference in light absorption and transmission by the sample. However, this method lacks contrast for live specimens, which is why dark-field and other illumination techniques are often chosen for such applications.