False. When using the 40X objective, you should not use the coarse adjustment knob.
Step-by-step explanation:
The coarse adjustment knob is used with lower power objectives if used at high powers the lens may get broken and also it would be difficult to attain desired focus. The coarse adjustment knob is used to attain the focus of the object by maintaining the distance between the object and the lens. This is the rough focus knob of the microscope and the focus of the object moves very quickly when using coarse adjustment.