Necromancy

Necromancy, communication with the dead, usually in order to obtain insight into the future or to accomplish some otherwise impossible task. Such activity was current in ancient times among the Assyrians, Babylonians, Egyptians, Greeks, Romans, and Etruscans; in medieval Europe it came to be associated with black (i.e., harmful, or antisocial) magic and was condemned by the church

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Its practitioners were skilled magicians who used a consecrated circle in some desolate spot, often a graveyard, to protect themselves from the anger of the spirits of the dead. In the event of a premature or violent death, the corpse was thought to retain some measure of unused vitality, and so the use of parts of corpses as ingredients of charms came to be an important technique of witchcraft. Necromancy was especially popular in the Middle Ages and Renaissance, and its temptations and perils were vividly described in the Faust stories of Christopher Marlowe and Johann Wolfgang von Goethe

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Microscope

Microscope, instrument that produces enlarged images of small objects, allowing the observer an exceedingly close view of minute structures at a scaleconvenient for examination and analysis. Although optical microscopes are the subject of this article, an image may also be enlarged by many other wave forms, including acoustic, X-ray, or electron beam, and be received by direct or digital imaging or by a combination of these methods. The microscope may provide a dynamic image (as with conventional optical instruments) or one that is static (as with conventional scanning electron microscopes).

The magnifying power of a microscope is an expression of the number of times the object being examined appears to be enlarged and is a dimensionless ratio. It is usually expressed in the form 10× (for an image magnified 10-fold), sometimes wrongly spoken as “ten eks”—as though the × were an algebraic symbol—rather than the correct form, “ten times.” The resolution of a microscope is a measure of the smallest detail of the object that can be observed. Resolution is expressed in linear units, usually micrometres (μm).

The most familiar type of microscope is the optical, or light, microscope, in which glass lenses are used to form the image. Optical microscopes can be simple, consisting of a single lens, or compound, consisting of several optical components in line. The hand magnifying glass can magnify about 3 to 20×. Single-lensed simple microscopes can magnify up to 300×—and are capable of revealing bacteria—while compound microscopes can magnify up to 2,000×. A simple microscope can resolve below 1 micrometre (μm; one millionth of a metre); a compound microscope can resolve down to about 0.2 μm

mages of interest can be captured by photography through a microscope, a technique known as photomicrography. From the 19th century this was done with film, but digital imaging is now extensively used instead. Some digital microscopes have dispensed with an eyepiece and provide images directly on the computer screen. This has given rise to a new series of low-cost digital microscopes with a wide range of imaging possibilities, including time-lapse micrography, which has brought previously complex and costly tasks within reach of the young or amateur microscopist.

Other types of microscopes use the wave nature of various physical processes. The most important is the electron microscope, which uses a beam of electrons in its image formation. The transmission electron microscope (TEM) has magnifying powers of more than 1,000,000×. TEMs form images of thin specimens, typically sections, in a near vacuum. A scanning electron microscope (SEM), which creates a reflected image of relief in a contoured specimen, usually has a lower resolution than a TEM but can show solid surfaces in a way that the conventional electron microscope cannot. There are also microscopes that use lasers, sound, or X-rays. The scanning tunneling microscope (STM), which can create images of atoms, and the environmental scanning electron microscope (ESEM), which generates images using electrons of specimens in a gaseous environment, use other physical effects that further extend the types of objects that can be examined.

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What Mad Pursuit: A Personal View of Scientific Discovery

What Mad Pursuit: A Personal View of Scientific Discovery

is a book published in 1988 and written by Francis Crick, the English co-discoverer in 1953 of the structure of DNA.