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Oil immersion Objective : In case of oil immersion objective the space between he outer lens of the objective and the object is fined with an oil like cedar wood oil which has the same refractive index as that of glass (1:51). The ray of light do not undergo refraction but pass into the objective giving a bright, clear image. For critical study of bacterial, blood cells etc., wherein high magnification and resolution is require, the oil immersion lens is of much help. Definition is the capacity of the objective to render the image distinct, eliminating chromatic and spherical aberrations. Spherical Aberration : The rays passing through the periphery of the lens focus on the axis at the shorter distance than those passing through the central portion of the lens with the result that the image is distorted