<P> In light microscopy, oil immersion is a technique used to increase the resolving power of a microscope . This is achieved by immersing both the objective lens and the specimen in a transparent oil of high refractive index, thereby increasing the numerical aperture of the objective lens . </P> <P> Immersion oils are transparent oils that have specific optical and viscosity characteristics necessary for use in microscopy . Typical oils used have an index of refraction around 1.515 . An oil immersion objective is an objective lens specially designed to be used in this way . Many condensers also give optimal resolution when the condenser lens is immersed in oil . </P> <P> Lenses reconstruct the light scattered by an object . To successfully achieve this end, ideally, all the diffraction orders have to be collected . This is related to the opening angle of the lens and its refractive index . The resolution of a microscope is defined as the minimum separation needed between two objects under examination in order for the microscope to discern them as separate objects . This minimum distance is labelled δ . If two objects are separated by a distance shorter than δ, they will appear as a single object in the microscope . </P>

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