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 ?
Uptake and depletion of plasma 17[alpha]-methyltestosterone during induction of masculinization in muskellunge, Esox masquinongy: effect on plasma steroids and sex reversal.
Park IS, Kimb JY, Choa SH, Kim DS (2004) Sex differentiation and hormonal sex reversal in the bagrid catfish Pseudobagrus fulvidraco (Richardson).
Induced sex reversal in tilapia nilotica (cichlidae) with methyl testosterone.
2006) also successfully induced sex reversal in medaka (Oryzias latipes) by immersion in 30 ng.
Autosomal XX Sex reversal caused by duplication of SOX9.
To begin addressing these questions, our objectives in the present study were 2-fold: to examine the role of brain aromatase in o,p'-DDT-induced sex reversal in medaka and to determine if an environmentally relevant mixture of both estrogenic (o,p'-DDT) and apparent antiestrogenic (TBT) chemicals, which have opposing effects on aromatase activity, can block each other's effects on reproductive development.
We also conducted an experiment with captive snook to document sex reversal in individual fish.
The number of colony forming units was lower in the sex reversal phase compared to the fingerlings phase.
1999) proposed that the 9p-associated sex reversal in humans might be a deletion syndrome that requires the inactivation of both Dmrt1 and Dmrt2.
London, July 21 (ANI): Researchers have discovered a gene, which maintains male sex throughout life - a breakthrough that can help shed light on the origin of some human sex reversal disorders.