Final answer:
The correct answer is option 1) When using low power.
Step-by-step explanation:
The coarse adjustment should be used to focus when using low power. Initially, adjusting the focus with the coarse focusing knob is crucial because it provides the primary level of focus and is essential for getting the object into the field of view. After achieving general focus under low magnification, the fine focusing knob is used for precision focusing. At higher magnifications, such as with medium or high power, only minor adjustments with the fine focusing knob are typically necessary due to the microscope's parfocality.
This characteristic ensures that once an object is focused at one magnification, it remains nearly in focus when switching to another magnification.
Optimal illumination is vital for a clear image, especially when using higher power lenses that have smaller apertures. Adjusting the brightness can be done through several methods: adjusting the light intensity, moving the condenser lens closer or farther from the object, and controlling the iris diaphragm. It's recommended to start observing samples using the lower magnification objectives to properly adjust these settings as the working distance is greater and the illumination requirements are less demanding.