Reversal


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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 periodicals archive ?
The retrospective case series was conducted at the Aga Khan University Hospital (AKUH), Karachi, and comprised records of patients who underwent colostomy reversal from January 2003 to December 2011, Data of all patients aged >15 years with complete medical record and follow-up of at least six months was retrieved through international classification of disease version 9 (ICD-9) coding system.
We have developed a series of protocols that allow us to monitor the change in performance after a number of short reversal events and measure the loss in carbon due to corrosion during the test.
Though the new guidance is not prescriptive, except when the cost to make these changes exceeds $10 million, PHMSA strongly encouraged operators to submit a comprehensive written plan before beginning a conversion or reversal.
After the initial simple discrimination training, the children were exposed to repeated reversal training.
NASA also provided more concrete details on the impact of the sun's magnetic reversal.
The last time that Earth's poles flipped in a major reversal was about 780,000 years ago, in what scientists call the Brunhes-Matuyama reversal.
Factors affecting the pregnancy rate after microsurgical reversal of tubal ligation.
At the onset of feeding or when food concentration was increased, the rate of suspension feeding increased, with a pump being directly followed by a post-pump reversal and then another pump.
By not using reversal agents, or by using agents that work slowly, patient recovery from surgery is prolonged," said Professor Manfred Blobner from the Klinik fur Anasthesiologie der Technischen Universitat Munchen, Germany.
Yet the reversal that rises above all others to the point of stumbling block and foolishness is the cross (cf.
Reversal of the reversal update: Carroll spoke with Verle Sorgen, the Pacific-10 Conference supervisor of officials, about the lengthy delay and eventual decision to give Oregon a touchdown on a tipped pass.