When 100x microscope objective lens is being used, light refraction in the air is enough high to produce image alterations. Image distortions can be minimized by immersion oil because this liquid has a higher refractive index than air. However, light refraction is not usually noticeable when using 4x, 10x and 40x objective lenses. Moreover, this image will not be sharp after putting oil on the slide and going back to the 40x objective, it is for that reason that the 40x is called 'high dry' lens. In consequence, it is required to clean the immersion oil off the 40x objective. In general, it is required to to remove the oil and refocus off the slide and lens immediately after oil immersion (it can be done by using an oil-soluble solvent).
