Oil Immersion Microscope Objectives

The resolution of a microscope optical system is dependent on many factors. Microscope objectives have many attributes. One common attribute is the medium in which the objective is designed to be used. It may be designated as dry, oil immersion, water immersion or glycerine immersion. By far the most common are dry and oil immersion. One of the factors that determines the ultimate resolution of the microscope is the NA (numerical aperture) of the objectives. Objectives with numerical apertures higher than 1.00 require a unified medium for the light rays forming the image to pass through. Since air nominally has a refractive index of 1.00 and the specimen slides typically have a refractive index of about 1.5, a 100X objective with an NA of 1.25 or greater can not perform to its highest capability( i.e. resolution). This is where oil immersion objectives come to the rescue! Among the many characeristics of microscope immersion oils is refractive index. For use in the visible spectrum most of these oils have a refractive index of approximately 1.515. If a microscopist places a drop of this oil between the objective and the specimen surface it effectively becomes part of the glass slide and unifies the refractive index of the system. In critical viewing situations, an oil immersion condenser can be employed. In this instance, the condenser is "oiled" by placing a drop of the same immersion oil between the condenser and the slide bottom thus unifying the refractive index of the entire system and improving the resolution to a much higher level than a "dry" system would allow. Many microscope manufacturers including Nikon, Motic and Unico include appropriate immersion oil as part of their microscopes.