Magnification is the process of enlarging the apparent size, not physical size, of something. In microscopy, it is the ratio between the size of an image produced by the microscope and its actual size. Microscopes magnify thin specimens mounted on microscope slides. They are ideal for observing unicellular or very small organisms, cells, and cell structures. We will use the compound and dissecting microscopes many times over the course of the semester. It is important to familiarize yourself with microscope use.

The resolving power of a microscope is dependent on the numerical apertures of the optical lenses and the wavelength of light used to examine the specimen. It is the smallest distance between two points (measured in microns) that can be seen with the microscope.  If two small objects close together can be seen clearly as two distinct objects, a microscope is said to have high resolving power.

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Base: the bottom of the microscope, which supports the entire instrument. The stage plate is located directly on the base surface upon which a specimen is placed. The stage can have a removable black or white tile (that can be removed and cleaned) or it will have a light that will transmit light through the specimen.

Köhler illumination is the alignment of the image-forming light path and the illumination light path of the microscope. In this process the con-denser is centered and focused, thereby providing an evenly illuminated field of view and more importantly maximum resolution of the specimen

Stereo microscopes have low magnifications that can range from 2 to 100x depending on the microscope, and are designed for viewing whole objects like rocks, plants, flowers, and invertebrate organisms by reflecting light off the specimen, producing a 3-dimensional image. Sometimes there is a light located in the base of the microscope that will allow transmitted light.

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Objective lenses: the primary optical system which produces a magnified image of the specimen. There are typically four objective lenses attached to the nosepiece with the magnification of each objective is engraved on its side.

Microscopes  must  be  calibrated  so  accurate measurements can be made. To calibrate a microscope  both  an  ocular  and  a  stage  micrometer are used.

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Fine adjustment or fine focusing knob: the smaller knob towards the back of the instrument that is used to make small adjustments in the height of the stage for final focusing on a specimen. It is the only focusing knob used with high power objectives.

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Ocular lens or eyepiece: the secondary optical system that you look through. The ocular lens further magnifies (10x) the image and brings the light rays to a focal point. A binocular microscope has two ocular lenses and a monocular microscope has one ocular lens that sit on the adjustable binocular body. Binocular lenses can be adjusted to fit the distance between your eyes by gently pulling the oculars apart or by pushing them closer together.

Note: The microscope is now set to maximize resolution of  the  specimen.    If  you  adjust  the  condenser  height  to gain contrast or adjust light intensity you will sacrifice the resolution capability.  Use the aperture diaphragm and /or the illumination intensity to adjust contrast.

Depth of  Field: is determined by the distance from the nearest specimen plane in focus to that of the farthest plane also simultaneously in focus. The thickness of the optical section along the optical axis within which objects in the specimen plane are in focus. High-magnification objectives have a decreased depth of field. The reverse is true of low-magnification objectives Field of View:  the visible area seen through the microscope when the specimen is in focus. The greater the magnification the smaller the view. Focus:  a specimen is in focus at the desired magnification when the image seen through the ocular lens is sharp and clear.

The 100X objective lens is called an oil immersion lens because oil is placed between the lens and the microscope slide to increase resolution (i.e., the level of detail that can be observed in an image). Light bends when it passes from the glass slide to air because of differing refractive indices. A drop of immersion oil between the slide and lens eliminates this problem because the oil has the same refractive index as the glass slide. Never use the 100X  objective lens without oil and do not get oil on the 4X, 10X, or 40X lenses.

Focusing knob: the knob that allows you to focus on the object at each magnification by moving the stereo head up or down.

Lateral Resolution: point-to-point resolving power in the plane perpendicular to the optical axis. It is usually defined as the shortest distance between two lateral points on the specimen plane that can still be distinguished as separate entities.

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A compound microscope is a high power microscope that uses a compound lens system. Higher magnification is achieved by using two lenses rather than just a single magnifying lens. While the eyepieces and the objective lenses create high magnification, a condenser beneath the stage focuses the light directly into the sample. A compound microscope has multiple lenses: the objective lens (typically 4x, 10x, 40x or 100x) is compounded (multiplied) by the eyepiece lens (typically 10x) to obtain a high magnification of 40x, 100x, 400x and 1000x. The objective lenses of a compound microscope causes the orientation of the image of the specimen to be inverted compared to the orientation of the actual specimen which means that a specimen viewed through a compound microscope will look upside down and backwards compared to how the specimen is mounted on the slide.

Condenser: the lens located below the stage, which focuses light (from the illuminator) through the specimen being observed. Most microscopes have a movable condenser allowing its distance from the specimen to be adjusted using the condenser knob and condenser alignment screws.

Stage: the flat surface upon which the slide with your specimen is placed. Most microscopes have a stage finger assembly to hold the slide on the stage. The entire mechanism including the slide moves horizontally across the stationary stage (left/right and forward/back) using two stage adjustment knobs situated under the stage (variably on the left or right side, in front of the focusing knobs).

Diopter: compensates for focusing differences between your eyes, it is very important this is set correctly, in order to prevent eye strain.

Axial Resolution: point-to-point resolving power in the plane parallel to the optical axis. It is usually defined at the shortest distance between two longitudinal points on the specimen plane that can still be distinguished as separate entities.

Coarse adjustment or coarse focusing knob: the large knob towards the back of the instrument that is used to significantly raise or lower the stage, when you first focus on a specimen at low power. It is never used when high power objectives are in place.

Illuminator or light source: the light source can be built into the base of the microscope, transmitting light through the specimen and/or the light source may be above the specimen as incident light. The lights can be turned on using rheostat (light) control knob on the front of the base.

Microscope are used by the students in many lab exercises. Instructors also need to learn to use the instructor microscope with the Leica camera and required LAS EZ & Leica AirLab Icon Guide software which will allow them to project the microscope images in real time.

Iris diaphragm: a unit below the condenser that controls the amount of light directed to the specimen. The diameter of the diaphragm can be adjusted by turning it to increase or decrease the size of the hole that light passes through.

Illuminator or light source: the light source is usually built into the base of the microscope, and directs light through the condenser to the specimen.Alternatively, the light source may be separate, and be directed toward the condenser with a mirror. The intensity of the light can be adjusted using the rheostat (light) control knob. The microscope you are using has a rheostat on the front of the base and a switch on the left of the base.