Oil Immersion Objectives
From the above it is understood that oil between the specimen and the objective lens improves the resolving power by a factor 1/n. Objectives specifically designed for this purpose are known as oil immersion objectives.
Oil immersion objectives are used only at very large magnifications that require high resolving power. Objectives with high power magnification have short focal lengths, facilitating the use of oil. The oil is applied to the specimen (conventional microscope), and the stage is raised, immersing the objective in oil. (In inverted microscopes the oil is applied to the objective).
The refractive indices of the oil and of the glass in the first lens element are nearly the same, which means that the refraction of light will be small upon entering the lens (the oil and glass are optically very similar). The correct immersion oil for an objective lens has to be used to ensure that the refractive indices are correctly matched. Use of an oil immersion lens with the incorrect immersion oil, or without immersion oil altogether, will suffer from spherical aberration. The strength of this effect depends on the size of the refractive index mismatch.
Oil immersion can generally only be used on rigidly mounted specimens otherwise the surface tension of the oil can move the coverslip and so move the sample underneath. This can also happen on inverted microscopes because the coverslip is below the slide.
Read more about this topic: Oil Immersion
Famous quotes containing the words oil and/or objectives:
“I bade, because the wick and oil are spent
And frozen are the channels of the blood....”
—William Butler Yeats (1865–1939)
“Along the journey we commonly forget its goal. Almost every vocation is chosen and entered upon as a means to a purpose but is ultimately continued as a final purpose in itself. Forgetting our objectives is the most frequent stupidity in which we indulge ourselves.”
—Friedrich Nietzsche (1844–1900)