But at higher magnifications, the distortion becomes so great that image details are lost. An oil immersion lens helps to remedy this problem by eliminating the air gap between the specimen and the objective lens. A drop of special immersion oil is placed on the microscope slide, and the oil immersion objective lens is maneuvered so that it is touching the oil. Immersion oil has the same refractive index as glass so that the light passes through the slide, specimen, oil and objective lens as if they were a single piece of glass. Changes in image composition coincide with changes in depth of focus.
Depth of focus is inversely proportional to magnification and aperture diameter.
Selected other prepared slides. Obtain a prepared slide of mixed bacteria. Mount the slide onto the stage of the microscope. Start with the lowest power objective in place. Using the course adjustment knob, move the objective lens to its lowest point.