Final answer:
Positioned at the subject's working distance means setting the microscope at the correct distance from the specimen for clear focus, which decreases with higher magnification and is easier with parfocal lenses.
Step-by-step explanation:
Being positioned at the subject's working distance means placing the microscope's objective lens at the correct distance from the specimen to achieve a clear focus. The working distance is the space between the objective lens and the specimen, and it decreases as you increase magnification.
Microscopes are usually designed to be parfocal, which means that minimal adjustments are needed when switching between lenses of different magnifications once the initial focus is set with a lower power lens.