How does a microscope work?
A microscope is an instrument that produces a clear magnified image of an object viewed through it. A microscope must be able not only to magnify objects sufficiently but also to resolve, or separate, the fine details of the object that are of interest to the viewer. In the optical microscope visible light rays, reflected from or transmitted by the viewed object, pass through a series of lenses and form an enlarged image of the object. This image is produced at the normal distance of clearest vision, which is about 10 inches, or 25 centimetres, from the eye of the viewer.
The degree to which the fine details of an object can be resolved is limited by the wavelength of the light that is used. Thus, finer details can be resolved if ultraviolet light is used instead of visible light, and even more detail can be resolved by using X rays. The greatest magnification and resolution are provided by the electron microscope. In this device a beam of electrons is used instead of rays of visible light for examining the object.
The basic form of microscope is the simple optical microscope. It consists of a single convex lens, or magnifying glass, or of a combination of lenses that has the same effect. The compound microscope, which is the most common type of optical microscope, consists of two separate lens systems. Each system is a simple microscope and may be either a single lens or a group of lenses. The object being viewed is placed close to one lens system, called the objective, which forms an image, called the primary image. This image in turn is the object that is magnified by the second system of lenses, called the eyepiece. The image seen by the eye is produced by the eyepiece at the normal distance of clearest vision.
Many compound microscopes are fitted with easily interchangeable objective lenses. They are mounted on a rotary device called the nosepiece, so that the lens can be replaced by another simply by turning the mounting until the replacement lens is in position.
The object to be examined is generally prepared in the form of a very thin slice of material. It is placed on a thin glass plate about 3 inches by 1 inch (7 by 2.5 cm), called a slide, and is covered by a thin glass sheet called a cover slip. The slide is set on the stage and illuminated from below. The light from a lamp is reflected in a mirror and passed through a lens called a condenser to concentrate it on the slide. This ensures that the object is evenly illuminated and that as much as possible of the light coming from the object enters the objective of the microscope. If the object is opaque or is unsuitable for mounting on a slide, a special microscope is used in which the object is illuminated from above.
The microscope is brought to a focus on the object by moving the body tube containing the lenses. The whole tube is moved up or down as required, but the relative position of the lenses in the tube is not altered.
Small living biological specimens may also be examined using a compound microscope. A drop of liquid containing the specimen is placed on a cover slip. The slip is then inverted and placed on a microscope slide that has a special depression at the centre. This method is called the hanging-drop method because the drop hangs in the depression in the slide. Specimens may be stained to increase their visibility.
The magnification produced by a compound microscope is the product of the magnification of the objective and the magnification of the eyepiece. Laboratory microscopes are fitted with a standard eyepiece that has a magnification of 10 times (usually written 10X). This eyepiece may be used in conjunction with the standard objectives 3.5X, 10X, and 40X. The respective overall magnifications would thus be 35X, 100X, and 400X.
A microscope consists essentially of a base, an arm, and a body tube. The principal optical parts of the microscope, the eyepiece and the objective, are mounted inside the body tube. The base of the microscope supports the stage on which the specimen to be studied is placed and also supports the arm on which the body tube is mounted.
History of the Microscope
A few hundred years ago, man became conscious of new worlds. He became aware of the universe of which this planet is a part, and the living microscopic universe filled with multitudes of creatures, the existence of which had been unknown. Thanks to the genius of two Dutch opticians, the telescope and the microscope were invented.
It took the imagination of the Italian, Galileo, to use their invention of the telescope to study the heavens. As a result, it was necessary to discard many of the theories that man had held as true for hundreds of years. It took the power of keen observation on the part of such men as Hooke and Grew of England, Malpighi in Italy, and Swammerdam and Leeuwenhoek in Holland to investigate the details of animal and plant structure that had not been visible before.
Anton van Leeuwenhoek (1632-1723) was the first to discover the living microscopic world. It is interesting, indeed, to note that his vocation was that of a linen merchant. His leisure time was spent in such amusements as glassblowing, metal work and grinding minute but powerful lenses. Until the 17th century, lenses had been used for various purposes, but they were all of low magnification. Actually, as long ago as 1294, Roger Bacon had experimented with lenses.
Leeuwenhoek used his powerful lenses to examine all sorts of things, such as saliva, plant leaves, blood from a salamander tail, teeth scrapings and pond water. He observed many forms of living creatures- those which we know as protozoa and bacteria. He called them "animalcules" and carefully recorded them for the Royal Society of London. Here, then, was definitely a case where the hobby of a man became of very great scientific importance.