The part of a microscope that allows one to change from low power to high power objective lenses is called the revolving nosepiece (or turret). This component holds the various objective lenses and allows the user to easily switch between them by rotating.
Regarding the use of the coarse adjustment knob, it is generally not advisable to use it when viewing objects with a high power objective lens for the following reasons:
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Risk of Damage: The coarse adjustment knob moves the stage (or objectives) significantly. If the stage is raised too quickly with a high power lens in place, the lens may come too close to the slide. This can cause the lens to hit the slide, potentially damaging both the lens and the slide.
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Loss of Focus: High power objective lenses have a very shallow depth of field, meaning they require precise focusing. Coarse adjustments can move the stage too much and can lead to overshooting the focal point, resulting in a loss of focus.
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Stability and Clarity: High power lenses typically require fine adjustments for clarity. Using the fine adjustment knob allows for more control and stability while focusing, which is essential for clear and precise viewing at high magnifications.
In summary, when using high power objectives, it's best to start with coarse adjustments at lower magnifications and then switch to fine adjustments for clarity and safety.