When looking through a compound light microscope which direction does the image move when you move the slide toward you and to the right?

Cells vary in size. With few exceptions, individual cells cannot be seen with the naked eye, so scientists use microscopes (micro = “small”; scope = “to look at”) to study them. A microscope is an instrument that magnifies an object. Most photographs of cells are taken with a microscope, and these images can also be called micrographs. The optics of a microscope’s lenses change the orientation of the image that the user sees. A specimen that is right-side up and facing right on the microscope slide will appear upside-down and facing left when viewed through a microscope, and vice versa. Similarly, if the slide is moved left while looking through the microscope, it will appear to move right, and if moved down, it will seem to move up. This occurs because microscopes use two sets of lenses to magnify the image. Because of the manner by which light travels through the lenses, this system of two lenses produces an inverted image (binocular, or dissecting microscopes, work in a similar manner, but include an additional magnification system that makes the final image appear to be upright). To give you a sense of cell size, a typical human red blood cell is about eight millionths of a meter or eight micrometers (abbreviated as eight μm) in diameter; the head of a pin of is about two thousandths of a meter (two mm) in diameter. That means about 250 red blood cells could fit on the head of a pin.

Most student microscopes are classified as light microscopes (Figure 1a). Visible light passes and is bent through the lens system to enable the user to see the specimen. Light microscopes are advantageous for viewing living organisms, but since individual cells are generally transparent, their components are not distinguishable unless they are colored with special stains. Staining, however, usually kills the cells.

Light microscopes commonly used in the undergraduate college laboratory magnify up to approximately 400 times. Two parameters that are important in microscopy are magnification and resolving power. Magnification is the process of enlarging an object in appearance. Resolving power is the ability of a microscope to distinguish two adjacent structures as separate: the higher the resolution, the better the clarity and detail of the image. When oil immersion lenses are used for the study of small objects, magnification is usually increased to 1,000 times. In order to gain a better understanding of cellular structure and function, scientists typically use electron microscopes.

Figure 1. (a) Most light microscopes used in a college biology lab can magnify cells up to approximately 400 times and have a resolution of about 200 nanometers. (b) Electron microscopes provide a much higher magnification, 100,000x, and a have a resolution of 50 picometers. (credit a: modification of work by “GcG”/Wikimedia Commons; credit b: modification of work by Evan Bench)

In contrast to light microscopes, electron microscopes (Figure 1b) use a beam of electrons instead of a beam of light. Not only does this allow for higher magnification and, thus, more detail (Figure 2), it also provides higher resolving power. The method used to prepare the specimen for viewing with an electron microscope kills the specimen. Electrons have short wavelengths (shorter than photons) that move best in a vacuum, so living cells cannot be viewed with an electron microscope. In a scanning electron microscope, a beam of electrons moves back and forth across a cell’s surface, creating details of cell surface characteristics. In a transmission electron microscope, the electron beam penetrates the cell and provides details of a cell’s internal structures. As you might imagine, electron microscopes are significantly more bulky and expensive than light microscopes.

Figure 2. (a) These Salmonella bacteria appear as tiny purple dots when viewed with a light microscope. (b) This scanning electron microscope micro graph shows Salmonella bacteria (in red) invading human cells (yellow). Even though subfigure (b) shows a different Salmonella specimen than subfigure (a), you can still observe the comparative increase in magnification and detail. (credit a: modification of work by CDC/Armed Forces Institute of Pathology, Charles N. Farmer, Rocky Mountain Laboratories; credit b: modification of work by NIAID, NIH; scale-bar data from Matt Russell)

For another perspective on cell size, try the HowBig interactive at this site.

The microscopes we use today are far more complex than those used in the 1600s by Antony van Leeuwenhoek, a Dutch shopkeeper who had great skill in crafting lenses. Despite the limitations of his now-ancient lenses, van Leeuwenhoek observed the movements of protista (a type of single-celled organism) and sperm, which he collectively termed “animalcules.”

In a 1665 publication called Micrographia, experimental scientist Robert Hooke coined the term “cell” for the box-like structures he observed when viewing cork tissue through a lens. In the 1670s, van Leeuwenhoek discovered bacteria and protozoa. Later advances in lenses, microscope construction, and staining techniques enabled other scientists to see some components inside cells.

By the late 1830s, botanist Matthias Schleiden and zoologist Theodor Schwann were studying tissues and proposed the unified cell theory, which states that all living things are composed of one or more cells, the cell is the basic unit of life, and new cells arise from existing cells. Rudolf Virchow later made important contributions to this theory.

Figure 3. These uterine cervix cells, viewed through a light microscope, were obtained from a Pap smear. Normal cells are on the left. The cells on the right are infected with human papillomavirus (HPV). Notice that the infected cells are larger; also, two of these cells each have two nuclei instead of one, the normal number. (credit: modification of work by Ed Uthman, MD; scale-bar data from Matt Russell)

Have you ever heard of a medical test called a Pap smear (shown in Figure 3)? In this test, a doctor takes a small sample of cells from the uterine cervix of a patient and sends it to a medical lab where a cytotechnologist stains the cells and examines them for any changes that could indicate cervical cancer or a microbial infection.

Cytotechnologists (cyto = “cell”) are professionals who study cells via microscopic examinations and other laboratory tests. They are trained to determine which cellular changes are within normal limits and which are abnormal. Their focus is not limited to cervical cells; they study cellular specimens that come from all organs. When they notice abnormalities, they consult a pathologist, who is a medical doctor who can make a clinical diagnosis.

Cytotechnologists play a vital role in saving people’s lives. When abnormalities are discovered early, a patient’s treatment can begin sooner, which usually increases the chances of a successful outcome.

A cell is the smallest unit of life. Most cells are so tiny that they cannot be seen with the naked eye. Therefore, scientists use microscopes to study cells. Electron microscopes provide higher magnification, higher resolution, and more detail than light microscopes. The unified cell theory states that all organisms are composed of one or more cells, the cell is the basic unit of life, and new cells arise from existing cells.

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Answer the question(s) below to see how well you understand the topics covered in the previous section. This short quiz does not count toward your grade in the class, and you can retake it an unlimited number of times. Use this quiz to check your understanding and decide whether to (1) study the previous section further or (2) move on to the next section.

Familiarization  First, familiarize yourself with all the parts of a microscope so that you can easily move from one part to another during operation. 

Moving and Placement: It is worth remembering that while a good quality microscope will last a lifetime, it is a sensitive scientific instrument that will suffer damage from sharp blows or impact. Always, therefore, carry your microscope in both hands. Grasp the arm with one hand and place the other hand under the base for support. Always place the microscope on a level and stable surface.

Slide Preparation: Microscope slides should always be prepared with a cover slip or cover glass over the specimen. This will help protect the objective lenses if they touch the slide. To hold the slide on the stage fasten it with the stage clips. You can push down on the back end of the stage clip to open it.

Focusing the Microscope:

•     Start by turning the revolving nosepiece (turret) so that the lowest power objective lens is "clicked" into position. The lowest power objective is the shortest one. This objective is the easiest to focus and center the image in the field of view.
•     While looking at the objective lens and the stage from the side, turn the coarse focus knob so that the stage moves upward toward the objectives. Move it as far as it will go without touching the slide.
•     Now, look through the eyepiece(s) and adjust the illuminator and diaphragm until you attain the maximum, comfortable level of light.
•     Slowly turn the coarse adjustment so that the stage moves down (away from the slide). Continue until the image comes into broad focus. The turn the fine adjustment knob, as necessary, for perfect focus.
•     Move the microscope slide until the image is in the center of the field of view. Then readjust the illuminator or diaphragm in order to attain the clearest image.
•     Once you have attained a clear image, you should be able to change to a higher power objective lens with only minimal use of the focusing adjustment. If you cannot focus on your specimen, repeat the above steps and work from objective to objective until the higher power objective lens is in place.

General Advice: You should check to see if your microscope has a rack stop. If it does not have one, then be careful not to allow the objective lens to touch the slide as you may break the slide. When using a monocular microscope, the correct technique is to look through the eyepiece with one eye and keep the other eye open. Most new users, tend to close one eye. While many microscopists do close one eye, you will help avoid eye strain by keeping both eyes open. Finally, remember! When you view a specimen through a microscope, you are viewing an image through multiple lenses. As a result, the image is upside down and back-to-front so when you move the slide to the right, the image moves to the left and vice versa!

Care & Maintenance of Your Microscope: Your compound microscope will last a lifetime if cared for properly and we recommend that you observe the following basic steps:

•     When finished viewing , lower the stage, click the low power lens into position and remove the slide.
•     Switch off the microscope when not using.
•     Avoid touching the glass part of the lenses with your fingers. Use only special lens paper to clean the lenses.
•     Dust is the number one enemy of a microscope so always keep your microscope covered when not in use. When not in use for extended periods, replace the microscope in its box.