Parts of a Light Microscope

Light Microscope

Parts of a Light Microscope

Eyepiece

Eyepiece (Ocular)

Eyepiece/Ocular lens is the lens you look through when you are using the microscope. Depending on the number of eyepieces, microscopes are classified as, monocular, binocular and trinocular microscopes. Usually has a magnification power of 10X or 15X.

Monocular: Microscopes with a single tube with one interchangeable eyepiece. The objects viewed through a monocular microscopes appear flat without any depth. The students microscopes are a good example for monocular microscopes and usually the cheapest.

Binocular: Microscopes with a single tube with two interchangeable eyepieces. The observer can use both eyes to examine the specimens. Binocular microscope eyepieces are equipped with a diopter adjustment for one/both eyepieces. Diopter adjustment allows you to change the focus on one eyepiece to compensate for the difference in vision between your eyes.

Trinocular: Microscopes with a single tube and two eyepieces and a third tube for a camera attachment. In order to capture images, you have to attach a compatible camera to this third tube. Usually research microscopes are trinocular microscopes and are the most expensive.

Monocular Eyepiece

Binocular Eyepiece

Trinocular Eyepiece

Body Tube

The tube connects the eyepiece to the objective lenses. The head part of the tube of the microscope is fixed to the microscope frame using a clamping screw. By loosening this screw, you can rotate the head 360° according to your preference.

Revolving Nosepiece - front view

Revolving Nosepiece- side view

Revolving Nosepiece

The Nosepiece can hold two or more objective lenses. In research microscopes , nosepiece can hold up to six objectives. Selection of the Objective lens can be done by revolving the nosepiece. To perfectly align the objective with the eyepiece, you have to rotate the revolving nosepiece until you hear a 'click' sound.

Objective Lenses of a Student Microscope

Objective Lenses of a Research Microscope

Objective Lenses

The objective lens is the closest component to the object. Objective lenses come in various magnifications (1X-150X). In most student microscopes, you will find 3-4 objective lenses. Usually come with scanning (4X), low power (10X), high power (40X) and oil immersion (100X) objective lenses. Objective lenses which are not used with an immersion medium are called "dry" lenses and those used with an immersion medium are called "immersion" lenses. Dry lenses should not be used with immersion medium as the oil can damage the protective coatings of dry lenses.

With increasing magnification, the length of the objective lens setup increases. Depending on the type of objective, the number of optical elements can vary from 2 to 15 or more. Objective lenses are parfocal, hence once you focus your specimens with low power, you don't have to refocus a lot when you switch objectives. However, fine adjustment is required to obtain a sharp crisp image.

Scanning Objective

Scanning Objective

This is the shortest objective with a magnification power of 4X. Usually low power objective is used to scan the specimen as you will be able to see all or most of the specimen.

Low Power Objective

Low Power Objective

Low Power objective has more magnification power than the scanning objective. Usually has a magnification power of 10X or 20X. Low power objectives are useful in observing most of the features of a specimen.

High Power Objective

High Power Objective

Usually has a magnification power of 40X or 60X. The high power (>40X) objectives has a small working distance, thus there is risk of the operator accidently hitting the objective lens on the slide. Therefore, high power objective are equipped with a spring loaded nosecone assembly that protects the front lens element from collision damages. If the front of the objective get connected with the slide, it will push into the objective barrel, thus preventing excess pressure on the lens element. High power objective is useful in observing fine details of a specimen.

Oil Immersion Objective

Oil Immersion Objective

Oil immersion objective provides the highest magnification power (100X). The lens must be used with a drop of immersion oil to bridge the gap between the slide/coverslip and the front lens. The immersion oils used for this purpose are clear and has a high refractive index. Without the immersion oil, you will not be able to get a sharp focus or resolution and specimen will appear blurry. Oil immersion lenses are useful in observing specimens such as bacteria, blood cells, striations in skeletal muscles, cell division etc.,. The sample must be non motile and no thicker than few micrometers.

Information on the objectives

You can see that there are some numbers printed on the barrel of the objectives. The numbers on the example given here include the following information in the following order:

Magnification/Numerical aperture (NA)/Tube length/Thickness of the coverslip

Scanning objective: Magnification = 4X, NA = 0.10, Tube length = 160 mm, Thickness of the cover glass = 0.17 mm
Low power objective: Magnification = 10X, NA = 0.25, Tube length = 160 mm, Thickness of the cover glass = 0.17 mm
High power objective: Magnification = 40X, NA = 0.65, Tube length = 160 mm, Thickness of the cover glass = 0.17 mm
Oil immersion objective: Magnification = 100X, NA = 1.25, Tube length = 160 mm, Thickness of the cover glass = 0.17 mm

In addition to the magnification power in number, the color bands marked on the barrel of the objectives also represent the corresponding magnification. Therefore, even if the magnification number is facing away from you, you can still identify the correct magnification of the objective by referring to the color bands. The color bands red, yellow, light blue and white represent 4X, 10X, 40X and 100X magnifications, respectively.

In more advance specialized objectives, you will find lot more information, for instance optical corrections, working distance, specialized optical properties, immersion medium, objective screw threads, parfocal distance etc., are printed on the objectives. For more information, visit specifications of objectives.

Mechanical Stage

Simple/Fixed Stage

Circular Stage

Stage

This is the platform where you place the slide. The microscope stages can be either Mechanical, simple/fixed or circular type.

Mechanical Stage

The mechanical stages are movable in x-y direction. During focusing the microscope, the whole stage along with the condenser move up or down as you turn the focusing knobs. The stage contains an elongated large opening in the middle to admit light coming from the condenser.

Simple/Fixed Stage

The fixed stages are not movable and come with spring loaded stage clips to hold the slide in position. If you need to adjust the field of view of the specimen, you have to manually adjust the slide. Manual adjustment of the slide under high power is not desirable as the slightest movement can bring the specimen out of the field of view.

Circular Stage

These stages can rotate 360°, permitting complete rotation of the specimen and great ease in observing the specimen, without remounting the sample. Circular stages can be locked into a position using the stage rotation lock. The circular stages also have two centering knobs to center the specimen under the objective lens. This option is useful to keep the specimen in the field of view, when rotating the specimen and shifting to a higher magnification.

Slide Holder

Stage clips/Slide holder

Slide holder holds the slide in place. The slide holder is spring loaded and holds and locks the slide in place. When slide is correctly placed in the holder, you can move the slide in both x (left and right) and y (Back and forth) directions using the stage controls located on the righthand side.

Stage Control

Stage Control

When you turn the bottom knob of the stage control, you can move only the the slide left and right, the stage remains in place. When you turn the top knob, you can move the stage back and forth. By using the stage controls you can examine a sample on any location of the slide.

Graduated Locator Markings

Graduated locator marks can be used to mark the location of specific items on a permanent slide with X-Y references. This allows you to return easily to the area for additional observation or photomicrography.

Top lens of the Condenser

Condenser and Iris Diaphragm

Condenser

The condenser is located under the stage directly below the stage opening. A simple Abbe condenser has two lenses and gathers light emitting from the microscope light source and concentrates it into a cone of light that illuminates the specimen with a uniform intensity over the entire view field. The condenser can move up or down using a lever or focus knob to control the lighting focus on the specimen. It is important to adjust the light cone to optimize the intensity and and angle of light entering the front lens of the objective. Each time you change the objective, corresponding adjustments to the condenser must be made to provide the proper light cone for the numerical aperture of the objective. .

Iris Diaphragm

Iris Diaphragm

The iris diaphragm is threaded to the bottom of the condenser. The opening and closing of the iris diaphragm can be done using the iris diaphragm lever. The opening and closing of the iris diaphragm will control the angle of illuminating rays which pass through the condenser and beyond. The iris diaphragm must be open to the correct position to match with the numerical aperture of the objective lens.

Light Source

Light Switch and Brightness Adjustment

Light Source

Many modern microscopes are equipped with a built-in-light source. Nowadays, most student microscopes come with an LED light source with external power supply. Usually there is a dial switch to control the brightness of the LED lamp. Other than LED, illumination can also be achieved by Tungsten-Halogen lamps, Mercury-Arc lamps, metal-Halide lamps, etc.,. In older light microscopes, you will find flat and concave mirrors to reflect either light from sun or an external lamp. Some microscopes are equipped with a field iris diaphragm to control the amount of light that enters the sub-stage condenser.

Coarse and Fine Adjustments

Limit Stop

Coarse and Fine Adjustments

Coarse Adjustment: By rotating the coarse adjustment (Grey grooved dial in the figure) ) you can move the stage up or down to bring the specimen into focus. Only a partial rotation of the coarse adjustment knob will produce a large vertical movement of the stage. Therefore, the coarse adjustment should be used only with scanning or low power objectives and never with the high or oil immersion objectives.

Fine Adjustment: By rotating the fine adjustment knob (Blue dial in the figure), you can move the stage up or down in small increments and bring the specimen into sharp focus under low power and use for all focusing when using high power objectives.

Limit Stop (Rack stop): This is a safety feature in microscopes to prevent the user from accidentally ramming the high power objectives into the cover glass or glass slides and damaging the objective lens or specimen. This adjustment is located behind the stage and has a fine-thread screw with a lock nut. By adjusting this screw you can decide how close the objectives can get to the slide.

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