You are going to view your Staphylococcus aureus slide under oil immersion. You see small spots as you are focusing on the slide. How can you distinguish small spots that are part of the specimen on the slide from specks of dirt on the microscope lenses ?

This can be achieved by increasing the resolution of the objective lens of the microscope. To achieve this both the objective lens ( with the dirt)and the specimen are immerse in the oil. The type of objective lens used for the immersion is called optical immersion objective

The oil is first applied to the specimen mounted on the microscope stage.Thus by raising the stage to torch objective lens both the lens and the specimen are immerse in oil. The oil Increases the resolution of the objective lens.

Generally the high refractive indices of the oil and water is close. Therefore the oil increase angular range at which the incident light on the lens will be transmitted , and therefore. focus on the specimen. Thus the oil with the objective lens reduces the refraction of light so that as light incident on the specimen the high refractive index of the lens and oil reduces the diffraction of light, and therefore focusing on the specimen increase.

Consequently, by adjusting the focal length , the objective lens at high magnification is able to resolve the dirt on it and the spots on the specimen. That is distinguished between the dirt on it and spots on the specimen as two distinct images rather than one.