BIOL 1406

PreLab 3.8

What are the parts of the brightfield microscope?

Cytology is the study of cell structure and function. Because most cells are very small, they can only be seen when magnified with an instrument such as a microscope. Microscopes not only provide important information about cell structure, they also provide clues about how the cell works.

There are 2 basic types of microscopes: the light microscope (creates an image using a beam of light), and the electron microscope (creates an image using a beam of electrons.) The most common type of light microscope, and the one you will use in this lab, is the brightfield microscope. The brightfield microscope is an example of a compound microscope. That means light from the object you are viewing passes through two lenses before it reaches your eye.

Microscopes not only magnify the object you are viewing, they also provide increased resolution. Resolution is the ability to distinguish two points as separate points. For instance, if two points are very close together, they may appear to be a single spot. If you increase magnification without increasing resolution, the single spot will only look like a larger single spot, and will never resolve into two separate spots. The better the resolution, the sharper and crisper the image. The resolving power of the naked eye is approximately 0.1 mm, meaning that our eyes can distinguish two points that are 0.1 mm apart. A light microscope can improve resolution by as much as 1000-fold. In addition, discernment of cellular detail can be improved with the use of dyes that add color and contrast to subcellular structures.

Magnification of a Compound Microscope

Total magnification = Power of the Ocular lens x Power of the objective lens


A typical ocular lens has a magnification of 10x; what is the total magnification if the high power objective (43x) is in position?

Total magnification = Ocular (10x) x Objective (43x) = 430x

In this lab exercise, you will learn about the different parts of the brightfield microscope, how to care for the microscope, and the correct procedure for viewing specimens. You will then practice using the microscope to view both a wet mount and a prepared slide of the organism Euglena.

Study the location and the function of each of the following parts of a brightfield microscope:


Figure 3.1 Parts of a brightfield microscope.

  • Base - The part of the microscope that rests on the table is called the base.

  • Light Source - The illuminator for your microscope is built into the base and is controlled by an on/off switch.

  • Arm - The arm is a vertical support that connects the base of the microscope to the body tube.

  • Body Tube - The body tube is the part of the microscope that connects the oculars with the revolving nosepiece. Some of the microscopes in lab have a small knob that sticks out from the front of the body tube. This knob can be used to move a pointer that is visible when you look through the microscope.

  • Ocular lenses - The oculars are the lenses you look through. Some microscopes are monocular--that is, there is only one eyepiece to look through. Other microscopes are dual-viewing; this means there are two oculars--one pointing upwards and one pointing backwards. These microscopes allow two people to view the same specimen at the same time--each looking through a single eyepiece. Yet other microscopes are binocular--that is, there are two eyepieces side-by-side and you look through both of the eyepieces, as you would a pair of binoculars. Most ocular lenses have a 10X magnification.

  • Revolving Nosepiece and Objective Lenses - Attached to the revolving nosepiece (which is attached to the bottom of the body tube) are several lenses called objectives. Most light microscopes have objective lenses of 4 magnifications: scanning (short) -- normally 4x power, low power (medium) -- normally 10x power, high-power (long) -- normally 40 - 43x power, and oil immersion (longest) -- normally 100x power.

  • Stage - The surface on which you place your slide is called the stage. It contains a hole in the center called the stage aperture. Two stage clips or a slide clamp are used to hold the slide in place.

  • Substage - The area under the stage, called the substage, contains the diaphragm. In addition, some microscopes also have a condenser located under the stage.

    • Condenser - The condenser contains a series of lenses that focus light onto the specimen.

    • Diaphragm - The diaphragm is an adjustable light barrier built into the condenser that regulates the amount of light passing through the specimen. It is very important that the diaphragm be correctly adjusted in order to get the best possible image. Use the smallest opening that does not interfere with the field of view. The condenser and diaphragm assembly may be adjusted vertically with a knob projecting to one side. Proper adjustment often yields a greatly improved view of the specimen. This adjustment has been made for you, and you should consult your instructor if you think that it must be readjusted.

  • Focus Knobs - You can focus your microscope by using the coarse and fine adjustment knobs located on the sides of the arm. Note that the coarse adjustment knob is larger than the fine adjustment knob. The coarse adjustment is used for initial focusing and should be used ONLY when the scanning (4X) or low power (10X) objective is in alignment. The fine adjustment makes very slight changes, allowing precision focusing on low or high power.



The part of the microscope that rests on the table is called the

Check your answer.

A series of lenses that focus light onto the specimen is called the Check your answer.
Which focus adjustment knob should NEVER be used when the high power or oil immersion objectives are in alignment? Check your answer.
Which part of the microscope regulates the amount of light passing through the specimen?
Check your answer.
The lenses you look through are called Check your answer.
What is the magnification of the low power objective? Check your answer.

The surface on which you place your slide is called the?

Check your answer.
The ability to distinguish two points as separate points is called Check your answer.


Use the drag and drop exercise below to practice identifying the different
parts of the microscope.

Close this browser window to return to Blackboard and complete the practice quiz and assessment quiz.