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 adage of history being written by the victors holds a lot of truth, and by researching the reversal we found out about the stories from the 'losers.'
Episodes of reform reversals in these countries may offer many important lessons to help understand how the reform process evolves in a society, how domestic institutions work and interact with each other, how internal social norms evolve through social learning and interact with external social norms and how all these promote and anchor reforms or lead to their reversal.
The study, "A Case Series Detailing the Successful Reversal of the Effects of Mifepristone Using Progesterone," authored by prominent abortion pill reversal developer George Delgado and several other national and international medical researchers, appears in the current issue (Volume 33, Number 1, 2018) of the peer-reviewed journal Issues in Law & Medicine.
"Moreover, the market has already been expecting the reversal. When considering worries over emerging economies or aversion to risky assets, these have not notably increased recently.
Along the same lines, Dunis and Lequeux (2001) analyzed the information content of risk reversals for the pairs USDJPY, USDCHF, GBPUSD and AUDUSD, using daily data and multivariate time series methods, also finding little evidence for the usefulness of risk reversals in assessing the future evolution of exchange rates.
A relatively stronger present-day field helps explain why Earth hasn't had a geomagnetic reversal for 780,000 years, says geophysicist Peter Driscoll of the Carnegie Institution for Science in Washington, D.C.
Tete, who was exposed only to sessions with all of the stimulus pairs in a random sequence, met the criterion in three sessions during Reversals 1 and 2.
Canada's National Energy Board (NEB) doesn't have regulations pertaining to reversals or conversions, but there is a thorough review process that considers all stakeholders, garners their objections, and allows the operator to demonstrate compliance through publicized mitigation strategies.
([section]) The 68 (50%) organizations that collect reports of reversals documented 8,032 overdose reversals (range = 0--2,079; median = 10; mean = 118.1).
Washington, Dec 1(ANI): Magnetic pole reversal is a common phenomenon, which occurs on Earth, gradually over the millennia.
Now cast your eyes upon more than one hundred clusters of 4-to-8-1etter imposing, transposing, swapping, flip-flopping, rotating, relocating, reciprocating, stupendous, tremendous, end-over-endous fortunate reversals:
(1) Laparotomy is currently seen as the gold standard for fallopian tube reversal, and this was the case at Tygerberg Hospital from 1982.