What is a Compound Microscope

Most cells (and hence most unicellular microorganisms) are too small to be detected by the unaided human eye. In order to detect them, and to observe their form and structure, the use of a microscope is therefore essential. The function of the magnifying lens system of this instrument, interposed between the specimen and the eye, is to greatly increase the apparent angle subtended at the eye by objects within the microscopic field. In addition to this factor of magnification, two other factors, contrast and resolution, are of great importance. In order to be perceived through the microscope, an object must possess a certain degree of contrast with its surrounding medium; and in order to produce a clear magnified image, the microscope must possess a resolving power sufficient to permit the perception as separate objects of closely adjacent points in the image.

The development and improvement of the more complex compound microscopes is now employed. A modern compound microscope contains three separate lens systems. The condenser, interposed between the light source and the specimen, collimates the light rays in the plane of the microscopic field. The objective produces a magnified image of the microscopic field within the microscope; and the ocular further enlarges this image and enables it to be perceived by the eye.

Single lenses have two inherent optical defects. They fail to bring the whole microscopic field into simultaneous focus (spherical aberration), and they produce colored fringes around objects in the field (chromatic aberration). These defects can be largely eliminated by placing additional, correcting lenses adjacent to a primary magnifying lens. Consequently, both the ocular and objective lenses of a modern compound microscope are multiple ones, designed to minimize these aberrations.

Resolving Limit:

The physical properties of light set a fixed limit to the effective magnification obtainable with a light microscope. Because of the wave nature of the light, a very small object appears to be a disc, surrounded by a series of light and dark rings. Two adjacent  points can be distinguished as separate, or resolved, only if the rings surrounding then do not overlap. The distance between two points that can just be distinguished from one another is known as the resolving limit, and it determines the maximal useful magnification of the compound microscope

Parts Of Compound Microscope:

The compound microscope has two phases of magnification; one called the objective phase and the other the eye piece. It is the most common microscope found in the laboratory especially used by students. To view the specimen light and lenses adjusted above the specimen is used. Some microscope comes with a mechanism to provide illumination. The objective lens is adjustable for better viewing of the specimen.