Different kinds of microscopes
How does a microscope work
So you received a microscope for your birthday. Congratulations! It’s a wonderful gift. But what are you going to do with it? Will you treat it as a toy to be played with and then stuck away on a shelf? Or will your consider it an exciting tool that can open your eyes to the hidden wonders of the microscopic world?
If you hope to use your microscope to the greatest advantage, there is a lot to learn. How does it make things look better? What parts does it have? How do they work? What can you look at? How do you make a slide? These and other questions are answered in this article.
Parts of the Microscope
A microscope is an instrument used to observe small things. When a watchmaker examines the works of a watch with his loupe, he is using a type of microscope. So is the person who uses a reading glass to see the works in his newspaper. And a doctor counting blood cells in a drop of blood is also using a microscope.
Microscopes make small things appear larger than they really are. This is called magnification. Microscopes also enable us to se many details of structure invisible to an unaided eye. This is called resolution.
The parts of a microscope that magnify the image and resolve the details are called lenses. Microscopes that enlarge in one stage, with a single lens system, are called simple microscopes. Microscopes that enlarge in two successive stages by means if two separate lens systems are called compound microscopes. A student-type biological microscope is an example of a compound microscope. Keep in mind, however, that there are many different types of microscopes.
The parts of a microscope may be grouped into three categories:
1) Optical parts
2) Parts related to illumination
3) Mechanical parts
If you have a microscope in front of you, examine each part as it is described.
These are the lenses that enlarge the object, or magnify, and bring out the details, or resolve. The lenses are carried in a body tube that keeps them the proper distance apart and in a straight line from the object to your eye.
The set of lenses nearest the object is called the objective. It provides the first stage of magnification. Its magnifying power is usually engraved on the side of the objective. Your microscope may have two or more objectives of different powers – for example, 10x and 43 x. These objectives are carried in a revolving nosepiece that makes it easy to switch from one to the other. As you rotate the nosepiece, a click tells you when the objective locks into place.
The set of lenses next to which you put your eye is called the ocular, or eyepiece. It provides the second stage of magnification. Its magnifying power is usually engraved on its upper surface – for example, 10x.
Thus, an object that you examine is first enlarged by the eyepiece. The total magnification seen by you equals the product of the two separate magnifications. For example, if you use 43x objective and a 10x eyepiece, the total magnification is 43 multiplied by 10, or 430 times.
Parts related to illumination
You can only see something through a microscope if light from the object reaches your eye. Some microscopes come equipped with a lamp that sends light rays through the oject into the microscopic tube. Others have a movable mirror that catches light from a natural or artificial source and reflects it up to your eye. The mirror usually has two sides: a flat side, which is a plane mirror, and a curved side, which is a concave mirror. The flat side is used with natural light sources – sulight. The concave mirror may be used to concentrate light from artificial sources.
Most microscopes also have a special diaphragm to regulate the cone of light entering the tube. This may be a disk diaphragm or an iris diaphragm. A disk diaphragm is a metal disk with cutout circles of various sizes. The disk can be rotated to line up any desired circle with the body tube and, thus, with the lenses. The smaller the circle, the smaller the cone of light that enters the microscope. The iris diaphragm is similar to the iris of your eye. It can open wide or be closed down to a pinpoint.
These parts make it easy and convenient for you to use your microscope. There is a heavy base, an upright supporting pillar, a curved arm, and a platform called the stage. The object to be examined is placed over the hole in the stage and is fastened with stage clips. An inclination joint allows you to tilt the microscope at a convenient angle for comfortable viewing. Special adjustment wheels operate gears that raise and lower the body tube to focus the microscope. When the object is in clearest view, it is in focus. If the image is not clear, it is out of focus. There are usually two adjustment wheels: a large coarse-adjustment wheel, and a smaller fine-adjustment wheel.
Good lighting – an essential
In the absence of light your eyes are like those of a blind person. For you to see any object, light must be present. There are three common ways in which light may reach your eyes.
1) Reflected light: rays of light from an outside source bounce off the surface of the object and reach your eyes.
2) Transmitted light: rays of the light from an outside source pass through the object and reach your eyes.
3) Luminescence: rays of light produced by the object itself reach your eyes.
In microscope work we usually depend on transmitted light. The mirror directs a beam of light through the object on the microscope stage. The light then passes through the objective and goes up the tube and through the eyepiece to your eye. The resulting image is in various shades of light and dark against a bright, light background. This is known as bright-field illumination.
If the specimen on the stage is so thick that light cannot pass through it, your eye sees nothing but darkness through the eyepiece. But the lighting arrangement can be changed so that rays of light bounce off the surface of the specimen and reflect up to your eye. In this manner you get a surface view of the object: a bright, shining image against a black background. This is known as dark-field illumination.
Once you know how to light up the microscope field, you are ready to look at various specimens. But good microscope technique requires a lot of practice. So don’t rush into things. Begin slowly and make progress step by step.