Water-immersion lenses are also available. These use water as an immersion liquid and allow biologists to examine specimens in a watery medium without the burden of a cover slip confining the living organisms.

Early microscopes had as their condenser a single lens, which was fixed in the end of the instrument facing the lamp (as in barrel microscopes) or mounted below the stage (as in the Bancks microscopes used by Robert Brown, Charles Darwin, and others). More-complex designs followed, their development driven by the peculiarly English obsession of observing fine details on diatom frustules. Achromatic condensers followed, but they are more troublesome to use because they need precise focusing, and the working distance is short.

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The highest-power microscope objective available is the immersion objective. When this type of objective is used, a drop of oil must be placed between the object on the microscope slide and the objective. The oil used has an R.I. that matches that of the glass in the first component of the objective.

Lenses with high magnification power bend light rays much more than lenses with low magnification power. This makes the image of the object come into focus at a much shorter distance from the lens. Hence, lenses with high magnification power are placed much closer to the specimen under observation. Conversely, low-power lenses are placed further away from the prepared slide.

High-power objectives pose several design problems. Because the focal length of an objective decreases as the N.A. and magnifying power increase, the working distance, or distance from the front of the objective to the top of the slide, is shorter for higher-power objectives. The need to use additional elements in the lens system for high magnifications further shortens the working distance to only 10 to 20 percent of the focal length. Thus, a 40× objective of 4-mm (0.2-inch) focal length may have a working distance of less than 0.4 mm (0.02 inch), so objectives with an increased working distance have been designed. These use a negative lens element between the object and the eyepiece, which has the added attraction of providing some field flattening as well. These objectives are especially of value in use with video systems.

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Function of light switch inmicroscope

Manufacturers provide objective lenses with standard magnifications usually ranging from 2 to 100×. The focal length of the objective is inversely proportional to the magnification and, in the majority of modern microscopes, equals the tube length (usually 160 mm [6.3 inches]) divided by the magnification. The field of view of the eyepiece is usually set to be a standard size of about 20 mm (0.8 inch) diameter. The field of view of the objective is then set to range from 10 mm (0.4 inch) for an objective with a magnifying power of 2× to 0.2 mm (0.008 inch) for an objective with a magnification of 100×. As a result, the angular field of view is about 7° for all objectives.

The condenser is placed below the stage and concentrates the light, providing bright, uniform illumination in the region of the object under observation. Typically, the condenser focuses the image of the light source directly onto the plane of the specimen, a technique called critical illumination. Alternatively, the image of the source is focused onto the condenser, which is in turn focused onto the entrance pupil of the microscope objective, a system known as Köhler illumination. The advantage of the latter approach is that nonuniformities in the source are averaged in the imaging process. To obtain optimal use of the microscope, it is important that the light from the source both covers the object and fills the entrance aperture of the objective of the microscope with light.

Criticalillumination

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For objectives with magnifying powers of 10×, the required N.A. increases to 0.25, and a more complex type of lens is required. Most microscope objectives of this magnification use a separated pair of doublets that share the refractive power. The correction of spherical aberration is readily achieved, but residual chromatic aberration is obtained when normal optical glasses are used for the lens elements. For most optical applications this is not important, but for critical high-magnification objectives (magnifications greater than 25×) this aberration is visible as chromatic blur. The correction of this residual aberration is achieved through the use of special optical glasses whose dispersion properties vary from normal glasses. There are only a few such glasses or crystalline materials that are useful for this purpose. Objectives that use these special glasses are called apochromats and were first produced commercially by Abbe in the 1870s.

Diaphragm. Attached below the stage, it can be either a disc or iris diaphragm that controls the intensity of the light entering the microscope.

What isilluminationinMicroscope

The first component of immersion objectives is generally a hyper-hemisphere (a small optical surface shaped like a hemisphere but with a boundary curve exceeding 180°), which acts as an aplanatic coupler between the slide and the rest of the microscope objective. An immersion objective with a high N.A. typically consists of a hyper-hemisphere followed by one or two aplanatic collectors and then two or more sets of doublets. Such objectives are made with magnifying powers greater than 50×, the extreme being about 100×.

Magnification system ofmicroscope

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Compound light microscopes have their own light source (illuminator) attached to the microscope base. The light rays are focused onto the stage by the condenser, which is placed below the stage.

Microscope lenses use transmitted, refracted, or reflected light to increase the image size of the object being viewed. This aspect is called magnification. The magnification power of a microscope depends on the lenses’ ability to bend light waves.

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“Micro” means small and “scope” means to see or view. Thus, humans developed microscopes to observe small things that are invisible to the naked eye. The earliest optical microscopes were only a step up from a magnifying lens. They had simple scopes connected to a single lens. This resulted in limited magnification ability, and they were called simple microscopes.

Microscope illumination techniquespdf

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In objectives with magnifying powers of 25× or greater, meniscus-shaped aplanatic elements are designed into the microscope objective in the space between the object and the pairs of doublets that carry out the relayed imaging. These aplanatic components have the property of converging the light without adding spherical aberration to the image and provide an increase in the N.A. without introducing significant aberration.

The object’s image must not only be enlarged but also its minute details must be clearly visible to the viewer’s eye. This aspect is called resolution. The clarity of the image produced depends on the lens quality and the frequency of the light waves falling onto the specimen. This is because light rays with high frequency have short wavelengths, which improve the image resolution.

The eyepiece is selected to examine the relayed image under conditions that are comfortable for the viewer. The magnifying power of the eyepiece generally does not exceed 10×. The field of view is then about 40° total, a convenient value for a relatively simple optical design. The observer places the eye at the exit pupil of the eyepiece, the point at which the light rays leaving the eyepiece come together. In most cases an eye relief (or distance from the exit pupil to the last element of the eyepiece) of about 1 cm is desirable. Too short an eye relief makes viewing difficult for observers who wear corrective eyeglasses.

The N.A. and the complexity of the objective increase as the magnification increases. Low-power objectives, of order 2–5×, are generally two-element lenses. Ordinary crown glass and flint glass (optical glasses with, respectively, relatively low and high R.I.’s) can be used to correct for spherical and chromatic aberration.

In 1590, three Dutch spectacle makers invented an optical microscope with more than one lens. This was the first compound microscope. Today, compound microscopes are used in most research laboratories, hospitals, and schools. Read on to learn more about what a compound microscope is and how it works so that you can select the best type of microscope for you.

The large N.A. of a microscope objective restricts the focusing requirements of the objective. The depth of focus is shown in the table as the accuracy with which the focal plane must be located in a direction along the axis of the microscope optics in order that the highest possible resolution can be obtained.

Köhlerillumination

Magnification power is calculated as the extent of image enlargement performed by the lens. For example, a magnification power of 10X indicates an approximate 10-fold increase in the size of the object’s image. Because compound microscopes have multiple lenses, the total magnification achieved is a numeric multiple of the magnification powers of all the individual lenses.

The mirror is attached to the base of the microscope. It has a plain surface on one side and a concave surface on the other. The mirror reflects rays from an external light source into the microscope.

The illumination system of the standard optical microscope is designed to transmit light through a translucent object for viewing. In a modern microscope it consists of a light source, such as an electric lamp or a light-emitting diode, and a lens system forming the condenser.

The objectives described above are usually intended to project an image through an eyepiece for direct viewing by an observer. The use of a photographic recording method permits the capture of a real image in a film holder or digital imaging system without an eyepiece lens. One approach is to remove the eyepiece and place the film holder or digital camera in the focal plane of the eyepiece, thus intercepting the image from the objective directly. A better approach is to use a specifically designed projection eyepiece, which can be adjusted to provide the appropriate magnification coupling the image to the film. Such an eyepiece can incorporate a change in the chromatic aberration correction to accommodate the requirements of the image-capture system.

Adjustment knobs. There are two knobs for fine and coarse adjustment. They move the body tube to bring the object being studied into focus.

A compound microscope is a type of optical microscope that uses visible light and multiple lenses to help you observe a real and magnified image of tiny objects. Through a compound lens system, it produces enlarged images of microscopic objects like living and dead organisms, tissues, and cells.

Microscopebase function

Stage. A flat plate connected to the lower end of the curved arm. It has a hole (aperture) at the center that allows the passage of light. Specimens to be examined are mounted onto the stage.

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Microscopes have become a critical part of modern science laboratories, whether that may be while you’re still studying in schools and colleges or working at hospitals and research facilities.

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They’re attached to the nosepiece at the base of the body tube. They can be of many types — scanning (4X magnification), low power (10X magnification), high power (40X magnification), oil immersion (100X magnification), and specialty (2X, 50X oil, 60X, and 100X dry magnification).

The optics of the microscope objective are defined by the focal length, N.A., and field of view. Objectives that have been corrected for aberrations are further defined by the wavelength requirements and the tube length of the microscope.

Focusing inmicroscope

Conventional objectives do not produce a flat image surface. The field curvature is generally of little importance in the visual use of the microscope because the eye has a reasonable accommodative capability when examining the image. Field curvature is a problem for imaging systems, however. Special objectives with flat-field lenses have been designed for these systems.

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Body tube. A hollow tube attached to the upper end of the arm. The upper portion (draw tube) contains the eyepiece lens. The body tube length is usually around 6.3 inches or 160 millimeters.

When rays from an incandescent light source enter the microscope lenses, they’re not uniform. They’re travelling in multiple directions and have differing wavelengths. Once the light waves pass through the specimen and reach the objective lenses, they’re bent into parallel paths. The objective lens then projects an inverted but magnified image onto a fixed intermediate image plane within the microscope. The ocular lens further magnifies this projected image.

Apart from condensers that are matched to specialized objectives (such as phase-contrast systems), others are available for specific applications. Thus, the dark-ground, or dark-field, condenser illuminates specimens against a black background and is eminently applicable to the observation of structures such as bacteria and flagellated cells in water. The use of colour filters, pioneered in the closing years of the 19th century by British microscopist Julius Rheinberg and now known as Rheinberg illumination, allows one to practice a form of dark-ground microscopy in which the background and the specimen are in contrasting colours. Although this technique is of no diagnostic benefit, the results can be spectacularly beautiful.

Increasingly prevalent today is the use of an electronic detector such as a complementary metal-oxide semiconductor (CMOS) or charge-coupled device (CCD) chip to capture the magnified image as a digital signal. This signal can be transmitted to a computer and translated into an image on the monitor. Software allows the user to take single pictures, moving video sequences, or time-lapse sequences at the click of a mouse. These may be saved for conventional viewing, and image processing can be used to enhance the result. Analysis of area and particle size and distribution is easily done by conventional analytical means once the images have been digitally captured. The production of computer presentations, transmission via e-mail, and ease of printing are benefits that digital imaging brings to the modern microscopist.

The eyepiece or ocular lens is attached to the top of the body tube. The rim of the eyepiece contains markings such as 5X, 10X, 15X, and 20X, which denote its magnification power. Place your eye above this lens to observe the magnified image of the object placed on the stage.

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Lenses with high magnification power also tend to provide greater resolution. Here are the two main types of lenses in a compound microscope:

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Human beings have always been curious about their surroundings. But when people first began to study flora and fauna, their observation of nature was limited to the visible world. Once the concept of magnification was introduced, researchers discovered that there was more than meets the eye.