Which part of the microscope regulates the amount of light that comes through the specimen

A high power or compound microscope achieves higher levels of magnification than a stereo or low power microscope. It is used to view smaller specimens such as cell structures which cannot be seen at lower levels of magnification. Essentially, a compound microscope consists of structural and optical components. However, within these two basic systems, there are some essential components that every microscopist should know and understand. These key microscope parts are illustrated and explained below.

STRUCTURAL COMPONENTS

The three basic, structural components of a compound microscope are the head, base and arm.

• Head/Body houses the optical parts in the upper part of the microscope
• Base of the microscope supports the microscope and houses the illuminator
• Arm connects to the base and supports the microscope head. It is also used to carry the microscope.

When carrying a compound microscope always take care to lift it by both the arm and base, simultaneously.

OPTICAL COMPONENTS

There are two optical systems in a compound microscope: Eyepiece Lenses and Objective Lenses:

Eyepiece or Ocular is what you look through at the top of the microscope. Typically, standard eyepieces have a magnifying power of 10x. Optional eyepieces of varying powers are available, typically from 5x-30x. 

Eyepiece Tube holds the eyepieces in place above the objective lens. Binocular microscope heads typically incorporate a diopter adjustment ring that allows for the possible inconsistencies of our eyesight in one or both eyes. The monocular (single eye usage) microscope does not need a diopter. Binocular microscopes also swivel (Interpupillary Adjustment) to allow for different distances between the eyes of different individuals.

Objective Lenses are the primary optical lenses on a microscope. They range from 4x-100x and typically, include, three, four or five on lens on most microscopes. Objectives can be forward or rear-facing.

Nosepiece houses the objectives. The objectives are exposed and are mounted on a rotating turret so that different objectives can be conveniently selected. Standard objectives include 4x, 10x, 40x and 100x although different power objectives are available.

Coarse and Fine Focus knobs are used to focus the microscope. Increasingly, they are coaxial knobs - that is to say they are built on the same axis with the fine focus knob on the outside. Coaxial focus knobs are more convenient since the viewer does not have to grope for a different knob.

Stage is where the specimen to be viewed is placed. A mechanical stage is used when working at higher magnifications where delicate movements of the specimen slide are required.

Stage Clips are used when there is no mechanical stage. The viewer is required to move the slide manually to view different sections of the specimen.

Aperture is the hole in the stage through which the base (transmitted) light reaches the stage.

Illuminator is the light source for a microscope, typically located in the base of the microscope. Most light microscopes use low voltage, halogen bulbs with continuous variable lighting control located within the base.

Condenser is used to collect and focus the light from the illuminator on to the specimen. It is located under the stage often in conjunction with an iris diaphragm.

Iris Diaphragm controls the amount of light reaching the specimen. It is located above the condenser and below the stage. Most high quality microscopes include an Abbe condenser with an iris diaphragm. Combined, they control both the focus and quantity of light applied to the specimen.

Condenser Focus Knob moves the condenser up or down to control the lighting focus on the specimen.

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The microscope, as shown in Figure 3-1, is one of the most important instruments utilized by the microbiologist. In order to study the morphological and staining characteristics of microorganisms such as bacteria, yeasts, molds, algae and protozoa, you must be able to use a microscope correctly.

Figure 3.1. The light microscope. A modern light microscope. This is an example of the kind used in the teaching labs at the University of Wisconsin-Madison. The various parts of the microscope are labeled. Please take the time to become familiar with their names.

The compound microscope used in microbiology is a precision instrument; its mechanical parts, such as the calibrated mechanical stage and the adjustment knobs, are easily damaged, and all lenses, particularly the oil immersion objective, are delicate and expensive. Handle the instrument with care and keep it clean.

The microscope is basically an optical system (for magnification) and an illumination system (to make the specimen visible). To help understand the function of the various parts of the microscope, we will follow a ray of light as it works its way through a microscope from the light source, through the lenses, up to the eye. Figure 3-8 traces the path of light through the parts of the microscope

Figure 3.8. The path of light through a microscope. Modern microscopes are complex precision instruments. Light, originating in the light source (1), is focused by the condensor (2) onto the specimin (3). The light then enters the objective lens (4) and the image is magnified. Light then passes through a series of glass prisms and mirrors, eventually entering the eyepiece (5) where is it further magnified, finally reacing the eye.

First let us consider a primary feature of all microscopes, the light source. Proper illumination is essential for effective use of a microscope. A tungsten filament lamp usually serves as the source of illumination. If reflected illumination is used, a separate lamp provides a focused beam of light which is reflected upward through the condenser lenses by a mirror.

The light from the illuminating source is passed through the substage condenser. The condenser serves two purposes; it regulates the amount of light reaching the specimen and it focuses the light coming from the light source. As the magnification of the objective lens increases, more light is needed. The iris diaphragm (located in the condenser), regulates the amount of light reaching the specimen. The condenser also collects the broad bundle of light produced by the light source and focuses it on the small area of the specimen that is under observation.

Light then passes up through the slide and into the objective lens where the first magnification of the image takes place. Magnification increases the apparent size of an object. In the compound light microscope two lenses, one near the stage called the objective lens and another in the eyepiece, enlarge the sample. The magnifying power of an objective lens is engraved in the lens mount. Microscopes in most microbiology laboratories have three objective lenses: the low power objective lens (10X), the high-dry objective lens (40X) and the oil-immersion objective lens (100X). The desired objective lens is rotated into working position by means of a revolving nosepiece.

On both sides of the base of the microscope are the course and fine adjustment knobs, used to bring the image into focus. Rotation of these knobs will either move the specimen and the objectives closer or farther apart. The coarse adjustment moves the nosepiece in large increments and brings the specimen into approximate focus. The fine adjustment moves the nosepiece more slowly for precise final focusing. In some microscopes, rotation of the fine and course adjustment knobs will move the stage instead of the nosepiece.

Magnification alone is not the only aim of a microscope. A given picture may be faithfully enlarged without showing any increase in detail. The true measure of a microscope is its resolving power. The resolving power of the lens is its ability to reveal fine detail and to make small objects clearly visible. It is measured in terms of the smallest distance between two points or lines where they are visible as separate entities instead of one blurred image. The resolving power of the objective lens, engraved on the lens, allows us to predict which objective lens should be used for observing a given specimen. However, having good resolution in the microscope does not guarantee a visible image, the resolving power of the human eye is quite limited. Often further magnification is needed to obtain a good image.

When the oil-immersion objective lens is in use, the difference between the light-bending ability (or refractive index of the medium holding the sample) and the objective lens becomes important. Because the refractive index of air is less than that of glass, light rays are bent or refracted as they pass from the microscope slide into the air, as shown in Figure 3-9. Many of these light rays are refracted at so great an angle that they completely miss the objective lens. This loss of light is so severe that images are significantly degraded. Placing a drop of immersion oil, which has a refractive index similar to glass, between the slide and the objective lens decreases this refraction, and increases the amount of light passing from the specimen into the objective lens. This results in greater resolution and a clearer image.

Figure 3.9. Refraction of light at 100X. Light passing out of the slide, into the air, toward the objective lens is refracted, due to the different in refractive index between air and glass. While the bending cause by this difference is not important at 100X and 400X, at 1000X this refraction is problematic, causing blurring of the image and significant loss of light. Immersion oil has a refractive index very similar to that of glass. Placement of a drop of oil between the objective lens and the slide prevents the bending of light rays and clarifies the image. The blue dashed line represents a potential light ray if immersion oil is not present. The red dashed line represents a light ray if immersion oil is present.

The image of the specimen continues on through a series of mirrors and/or prisms that bend it toward the eyepiece. A further magnification takes place at the eyepiece producing what is called a virtual image. Total magnification is equal to the product of the eyepiece magnification and the objective magnification. Most often eyepiece lenses magnify 10-fold resulting in total magnifications of 100, 400, or 1000X, depending upon which objective is in place. Many modern microscopes will also have focusable eyepieces to compensate for differences between individuals and even between individual's eyes. The adjustment of these is important and is described below.

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What part of the microscope that regulates the amount of light passes through the specimen?

Which part regulates or control the amount of light?