Reversal


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Related to Reversal: Magnetic reversal, vasectomy reversal, Risk reversal, Reversal potential

Reversal

 

in photography, a chemical process used to produce a positive image directly on exposed photographic material. A visible image is produced on the material’s light-sensitive layer by the photochemical action of light and by photographic development. In this image, called a negative, the bright parts of the subject appear dark and the dark parts, light. The negative is developed but not fixed because the silver halide remaining undeveloped in the light-sensitive layer is used to obtain the positive image. After a bleaching bath to remove the metallic silver, the photographic material is exposed a second time and redeveloped, resulting in a positive image.

In reversal processing of color materials, the first development produces a negative silver image and the second development yields a positive image consisting of silver and dyes. After all the silver is removed by bleaching and fixing, only a dye image remains.

After each stage of treating photographic materials in solutions, the materials are rinsed in water to remove soluble substances that could contaminate subsequent solutions or damage the image during storage.

Reversal is called a one-step process to distinguish it from the two-step negative-positive process. Images obtained by the reversal process can be multiplied by printing on reversal cinematographic materials or by duplication. Reversal processing is widely used in professional, amateur, scientific, and instructional photography and cinematography.

REFERENCES

Iofis, E. A. Kinoplenki i ikh obrabotka. Moscow, 1964.
Iofis, E. A. Tekhnika fotografii. Moscow, 1973.

E. A. IOFIS


Reversal

 

(reverse), a change in the fundamental motion of a machine or its working parts to the opposite direction. Reciprocating engines are reversed either by distribution mechanisms (valves or slide valves), which can direct the working fluid to the engine cylinders in a specified sequence, or by an intermediate unit in the transmission that changes the direction of rotation of the driven shaft without altering the motion of the drive shaft. In aircraft engines, reversal (the creation of reverse thrust) is achieved by reversing the propeller blades or deflecting the stream of exhaust gases.

Ships with steam- and gas-turbine power plants use auxiliary turbines for reversal. Ships also use propellers with rotating blades for the same purpose. Electric motors can be reversed by changing the direction of the current in the exciting winding of a DC motor or by switching two phases of the stator winding in AC motors.

M. S. FESTENSHTEIN

References in classic literature ?
You must excuse me," he said, "but a sense of humor was always my undoing, and this reversal of our positions is a little odd, isn't it?
In Lilliput all the dimensions are scientifically computed on a scale one-twelfth as large as that of man; in Brobdingnag, by an exact reversal, everything is twelve times greater than among men.
Desire is a subject upon which, if I am not mistaken, true views can only be arrived at by an almost complete reversal of the ordinary unreflecting opinion.
Here the good woman pointed to the little window, whence her son might be seen sitting disconsolate in the tuneless groves; and again shook her head and wiped her eyes, and besought him, for the united sakes of both the young people, to exercise his influence towards the bright reversal of these dismal events.
Not with any sense worthy to be called a sense, of this dire reversal of the places of parent and child, but in a pitiful expostulation to be let off from a scolding.
Why, the gayest feather in Miss Monflathers's cap, and the brightest glory of Miss Monflathers's school, was a baronet's daughter--the real live daughter of a real live baronet--who, by some extraordinary reversal of the Laws of Nature, was not only plain in features but dull in intellect, while the poor apprentice had both a ready wit, and a handsome face and figure.
However, even though rare, there will be situations when reversal of anticoagulation is medically necessary, said Professor Harald Darius, lead investigator for the RE-VERSE AD clinical study in Germany, and Director of the Department of Cardiology, Vascular Medicine, Nephrology and Intensive Care Medicine, Vivantes Neukoelln Medical Centre, Berlin.
different stimulus pairs that are presented within a session), which permitted the investigation of reversal learning sets and functional class formation in preschool children (e.
Due to a couple of well-publicized reversal failures, the public is all too aware of what can potentially go wrong with a pipeline reversal.
A reversal of the sun's magnetic field is, literally, a big event," Phillips said, according to( http://www.
Lerner are co-directors of the Vasectomy Reversal Center of America and are among the top vasectomy reversal specialists in the United States.
Earth has settled in the last 20 million years into a pattern of a pole reversal about every 200,000 to 300,000 years, although it has been more than twice that long since the last reversal.