![]() above 1.0 requires an objective which uses oil or water between it and the cover slip. This typically includes 1) magnification (number with X) 2) numerical aperture N.A 3) tube length the objective is designed for (160mm, 170mm or infinity symbol for infinity systems, 4) special medium if other than dry or air (oil, water and so on) 5) optical correction if other than achromat (flat field, plan, apochromatic) and sometimes 6) thickness of the cover glass to be used if other than standard. Each objective barrel is inscribed with useful information. Magnification is increased or decreased as needed by rotating different objectives into position. ![]() Objectives are threaded onto the turret of the microscope, which typically holds 3-4 objectives. The objective lens is the centerpiece of a microscope optical system. The highest N.A objectives are objectives that combine immersion with sophisticated optical correction. will require the use of an oil or water objective, since these respective mediums reduce the amount of bending (refraction) as light enters and leaves the slide. limit of 1.0 and in practice, it is rare to achieve an N.A even close to that. Dry objectives (objectives that are used without oil, water or other special medium on the slide) have a theoretical N.A. The relationship between aperture, magnification, medium and degree of optical correction is known as an objective's numerical aperture or N.A. Aperture (diameter of the objective) is still important, since a larger objective will deliver more light to the specimen, but the medium also affects the width of the cone of light available for studying the specimen. In microscopes, due to their higher magnifications and the variety of mediums in which objectives are uses (oil, water, air) other factors must also be considered. ![]() In most optical instruments, aperture and optical quality are the main determinants of resolution (ability to show tow closely spaced objects as being separate) and hence performance. On a compound microscope, the nosepiece is the area of the microscope that holds the turret and objectives. This configuration does reduce the cost but it also produces more eye strain and fatigue over long observing sessions. This is a microscope head which offers only one eyepiece for observation. The typical mirror is two sided, plano-convex (flat, curved) to accommodate various light sources. A mirror requires light from an external source such as the sun or lamp and the mirror reflects the light source upward to the condenser specimen via a mirror located below the stage. This is the traditional lighting system used on toy and children's microscopes, though a mirror system is still a viable option for serious microscopes which are used in locations with no power supply.
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