Because the refractive indexes of the immersion oil and the glass microscope slide are the same; the oil prevents the refraction of the incoming light rays.
An invisible space is present between the slide and the lens that contains air.
When the light rays fall on the slide and lens they might get refracted due to air present between them.
If the rays get refracted then the diameter of the objective lens will expand further.
So in order to prevent the refraction of light, it is very important to lubricate the surface of the lens because without oil the light rays refract from their original path.
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