Final answer:
The correct distance between the two sets of lenses on a microscope is maintained by the microscope's mechanical tube, ensuring parfocality and appropriate working distance for clear focus.
Step-by-step explanation:
On a microscope, the two sets of lenses, which often refer to the ocular lens (eyepiece) and the objective lenses, are kept the correct distance apart by the microscope's mechanical tube. This tube serves as a structural element that maintains the proper alignment and spacing between the lenses. It's crucial for ensuring that the image remains in focus when switching between different power objective lenses due to a feature called parfocality. Parfocality minimizes the adjustments necessary when changing magnification; once a good focus is achieved at a lower power, only minor adjustments with the fine focus knob are needed for higher power objectives. Similarly, in optics, like in microscopes and telescopes, working distance refers to the space between the objective lens and the specimen, which decreases with increased magnification.