When using a microscope, what should a student do if they move the 100x objective out of place before adding oil?

The most appropriate action when the 100x objective has been moved out of place before oil is added is to rotate objectives in the opposite direction to avoid oil contamination. This is crucial for maintaining the cleanliness of the microscope and its components. Oil immersion techniques require the objective lens to be in close proximity to the slide to enable clear viewing without air gaps that can distort the image.

If one were to place the 100x objective back into position after the oil has been added (as suggested in the second choice), there is a high risk of contaminating both the lens and the oil with other debris or substances that may have been present on the objective lens or other parts of the microscope. This could degrade the quality of the observation and potentially result in damage to the lens.

Lowering the stage of the parfocal microscope or adding oil first is not advisable as these actions do not mitigate the risk of contamination while ensuring that the proper objective is in place before the oil is used.

Thus, the best practice involves reversing the direction of rotation when moving the objectives, which minimizes the chance of introducing contaminants and allows for a clean and effective oil immersion microscopy experience.