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, 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
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 closely.
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 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. Are you sure it can\’t be focussed? Objective lens like 40x or 45x normally have very near-focus (can be less than 1 mm) and also a shallow. To focus the slide at 40X objective; at first focus it with 10X objective. Then rotate the turnstile at nose-piece and bring 10X objective. ( In some lower quality microscope with the adjustment screws you may need to bring the main-portion of microscope upwards, distant from the slide.
Otherwise the long 40x lens may hit the slide or drag the coverslip. However good-quality microscopes doesn\’t have this problem). Now it is time to focus the 40X. To easily do that; at first bring the 40X lens tip just 1 or 2 mm distance from cover-slip. (Just like sometimes while a known-titration we blow the titrant from the burette quite high-rate upto certain value; and subsequently slow-down). Now take only the fine-adjustment screw and bring the objective backwards (and very carefully forwards. so that the lens doesn\’t hit the cover slip). Note-1. I didn\’t worked with 25X eyepiece; but I worked up to 15X eyepiece; that normally gives wonderful clear picture. But in some microscopes having expandable draw tube, higher magnification with similar 15X eyepiece I\’ve seen, but gradually high expansion of drawtube (and refocus of course) makes image gradually more blurry.
But focus was not impossible. You could also try eye-pieces of 10X and 15X values. Note-2 : Use thin and uniform cover-slip (such as Blue-star (trade-name)) and try to make your sections more thin. See if that cause any improvement. Note-3 : Do not use bare specimen (no-coverslip). it may damage the lens which is very close to specimen. Eyepiece: 25x – Objective lens: 100x –> not focused. The 100X objective lense is usually a. I. e. between the coverslip and 100X lens tip; the light pass through a specialized oil instead of air. What I\’ve seen if I do not use the oil; I do not get the focus even when the lens tip touches the coverslip. Note-1 : Cleaning of oil immersion lens: Do not use xylene (though some protocol says that). It damages the eyepiece, cause blur, and may make it permanently non-functional.
Most of our university-teachers forbidden strictly to use xylene for lens cleaning. Use a soft cotton cloth, at first moist, then dry (or alternatively wet and dry for several times). However cotton and xylene to be used to clean the coverslip quickly with minimal damage/friction of the microscopic specimen. Note-2 : If use oil; make its use neat and clean way. Do not contaminate oils to other lense. Note-3 : For common purpose we do not need the oil-immersion lens. It is used only to view very tiny things like chromosomes, bacteria, cell-wall ornamentation, pollen walls, fungal spores and such type of things. Average plant and animal cells\’ contours (and tissues) could be seen well under the 10X objective and 10X to 15X eyepiece. Sometimes students\’ microscope come without the 100X objective; the place for it on the turnstile kept closed with a screw-cap.