Microscope
Mr.Cool
A magnifying glass, an ordinary double convex lens having a short focal
length, is a simple microscope. The reading lens and hand lens are
instruments of this type. When an object is placed nearer such a lens
than its principal focus, i.e., within its focal length, an image is
produced that is erect and larger than the original object. The image
is also virtual; i.e., it cannot be projected on a screen as can a real
image.
Compound Microscopes
The compound microscope consists essentially of two or more double
convex lenses fixed in the two extremities of a hollow cylinder. The
lower lens (nearest to the object) is called the objective; the upper
lens (nearest to the eye of the observer), the eyepiece. The cylinder
is mounted upright on a screw device, which permits it to be raised or
lowered until the object is in focus, i.e., until a clear image is
formed. When an object is in focus, a real, inverted image is formed by
the lower lens at a point inside the principal focus of the upper lens.
This image serves as an "object" for the upper lens which produces
another image larger still (but virtual) and visible to the eye of the
observer.
Computation of Magnifying Power
The magnifying power of a lens is commonly expressed in diameters. For
example, if a lens magnifies an object 5 times, the magnification is
said to be 5 diameters, commonly written simply "5x." The total
magnification of a compound microscope is computed by multiplying the
magnifying power of the objective by the magnifying power of the
eyepiece.
Development and Uses
The invention of the microscope is variously accredited to Zacharias
Janssen, a Dutch spectaclemaker, c.1590, and to Galileo, who announced
his invention in 1610. Others are known for their discoveries made by
the use of the instrument and for their new designs and improvements,
among them G. B. Amici, Nehemiah Grew, Robert Hooke, Antony van
Leeuwenhoek, Marcello Malpighi, and Jan Swammerdam. The compound
microscope is widely used in bacteriology, biology, and medicine in the
examination of such extremely minute objects as bacteria, other
unicellular organisms, and plant and animal cells and tissue-fine
optical microscopes are capable of resolving objects as small as 5000
Angstroms. It has been extremely important in the development of the
biological sciences and of medicine.
Modified Compound Microscopes
The ultramicroscope is an apparatus consisting essentially of a
compound microscope with an arrangement by which the material to be
viewed is illuminated by a point of light placed at right angles to the
plane of the objective and brought to a focus directly beneath it. This
instrument is used especially in the study of Brownian movement in
colloidal solutions (see colloid). The phase-contrast microscope, a
modification of the compound microscope, makes transparent objects
visible; it is used to study living cells. The television microscope
uses ultraviolet light. Since this light is not visible, the apparatus
is used with a special camera and may be connected with a television
receiver on which the objects (e.g., living microorganisms) may be
observed in color.
Electron Microscopes
The electron microscope, which is not limited by the powers of optical
lenses and light, permits greater magnification and greater depth of
focus than the optical microscope and reveals more details of
structure. Instead of light rays it employs a stream of electrons
controlled by electric or magnetic fields. The image may be thrown on a
fluorescent screen or may be photographed. It was first developed in
Germany c.1932; James Hillier and Albert Prebus, of Canada, and V. K.
Zworykin, of the United States also made notable contributions to its
development. The scanning electron microscope, introduced in 1966,
gains even greater resolution by reading the response of the subject
material rather than the direct reflection of its beam. Using a similar
approach, optical scanning microscopes achieve a resolution of 400
Angstroms, less than the wavelength of the light being used. Finally,
the scanning tunnelling microscope, invented in 1982, uses not a beam
but an electron wave field, which by interacting with a nearby specimen
is capable of imaging individual atoms; its resolution is an astounding
one Angstrom.
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