Question

why should the fine focus knob typically be used with the 10x and 40x objective lenses?

the slide should be close to in focus after focusing at the 4x objective. focusing with the coarse focus knob would result in too large a change in focus.

the coarse focus knob is nonfunctional with the 10x and 40x objective lenses.

focusing using the coarse adjustment knob with the 10x or 40x objective may crack the stage.

Explanation:

Analyze the function of focusing knobs and parfocality

The concept of parfocality means that once an image is focused under a lower power objective (such as \(4\text{X}\)), it remains nearly in focus when switching to higher power objectives (such as \(10\text{X}\) and \(40\text{X}\)). Consequently, only minimal adjustments are needed to bring the specimen into sharp focus.

Evaluate the mechanical risks and precision requirements

At higher magnifications (\(10\text{X}\) and \(40\text{X}\)), the working distance between the objective lens and the slide is very small. Using the coarse focus knob makes large vertical movements of the stage, which can easily overshoot the focal plane or cause the objective lens to crash into the slide. Therefore, the fine focus knob must be used to make precise, minute adjustments.

Assess the given multiple-choice options

• Option 1 correctly states that the slide is already close to focus after using the \(4\text{X}\) objective, and that the coarse focus knob would cause too large a change in focus.
• Option 2 is incorrect because the coarse focus knob is still mechanically functional, though unsafe and impractical to use.
• Option 3 is incorrect because while a collision can damage the slide or objective lens, it does not "crack the stage."

Answer:

• (A) The slide should be close to in focus after focusing at the 4X objective. Focusing with the coarse focus knob would result in too large a change in focus. (Correct answer)
• (B) The coarse focus knob is nonfunctional with the 10X and 40X objective lenses.
• (C) Focusing using the coarse adjustment knob with the 10X or 40X objective may crack the stage.