Scanning Objective (4X): This shortest objective is useful for getting an overview of the slide (especially handy with some of the slides that contain whole organs like a section of the spinal cord, lung, digestive tract, ovary, etc.).

Low Power Objective (10X): This next shortest objective is probably the most useful lens for viewing slides.  Almost any feature you need to observe in this course can be located with the 100X total magnification this objective provides.  As an added benefit, the low power objective is always safe to use as it cannot be lowered to the point of contacting and thus possible breaking a slide.  This is also true of the scanning objective mentioned above.

High Power Objective (40x): This objective (sometimes called the "high-dry" objective) is useful for observing fine detail such as the striations in skeletal muscle, the arrangement of Haversian systems in compact bone, types of nerve cells in the retina, etc.

Oil Immersion Objective (100X): This longest objective is used for observing the detail of individual cells such as white blood cells, the cells involved in spermatogenesis, etc.  The lens must be used with a specially formulated oil that creates a bridge between the tip of the objective and the cover slip.  Since the refractive indices of air and this lens are different, the lens will not work without this special oil!