The lens of the oil mirror is very small, the light through the air between the slide and the oil lens, due to the density of the medium is different, refraction or total reflection occurs, so that the light emitted into the lens is reduced, the object image is not clear. If you add cedar oil (n=1.515), which has a refractive index similar to that of glass (n=1.52), between the oil mirror and the slide, the amount of light entering the lens increases and the brightness of the field of view is enhanced, resulting in a bright and clear image.
In the use of oil microscope, when using a low-magnification dry objective lens to select the observation area, people tend to underestimate the very small object field of the high-magnification objective lens, so much so that it may go to focus an empty field of view. Also the center of the objective is rarely extremely accurate, so the field of view of the high magnification objective is not likely to be estimated very accurately either. These make it very difficult to focus an oil lens. In such cases a very small piece of cigarette tinfoil or a very thin piece of tinfoil may be enclosed under a coverslip with the specimen, so that it is easy to use the oil glass to find a plane of focus which is very close to that of the specimen.
When using an oil lens, you can go back to the low-power objective without wiping off the immersion oil. The oil immersion layer does not greatly affect the image formation when using the low magnification objective, and the front lens of the objective does not come into contact with the immersion oil, while the immersion oil on the coverslip does not increase the effect of the coverslip on the image.
The first immersion oils used in Olympus microscopes were cedar oils with a refractive index of 1.512-1.518, which increase in wilt when exposed to air and have slow polymerization properties. Nowadays, many microscope manufacturers can provide a non-resinous synthetic immersion oil with a refractive index of 1.5 15-1.5 18 and suitable dispersion characteristics, which has gradually replaced the above two resinous immersion oils due to its good characteristics and suitability for high-quality oil immersion work. Therefore, it is recommended to use this type of mirror oil when using oil mirrors.
Only one drop of immersion oil is needed, too much immersion oil is not only unhelpful for observation, but also will bring a lot of trouble to the cleaning of the oil mirror after use. At the end of the observation, even if the use of non-resinous immersion oil should be carefully cleaned on the objective lens and coverslip, first wipe off the immersion oil with a dry eraser or dry cloth, and then wipe off the residual immersion oil with an eraser or cloth dipped in xylene or gasoline.
