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 discussion of the introduction and reversal of pension reforms in the region given below provides a good example in this regard, showing some of the typical characteristics of institutional monocropping, such as technocratic rule and institution transfer, albeit in many cases strongly supported by political processes.
Where the problems started to emerge, as we shall see in the discussions on the reform reversal episodes, was the lack of sufficient social learning following EU accession, and perhaps also the lack of a political process to promote and ease the adoption to the new rules and institutions, given that the formal reversal of individual elements protected by EU law is not a viable option.
The reversal of the anticoagulant effect of RIV may be required in emergency situations, such as in subjects with a life-threatening and serious bleeding event or in the case of urgent surgery.
2 are used to restore physiological hemostasis in patients utilizing an anticoagulant including (1) enhancing the rate of drug clearance (elimination) (2) hemostatic agents (reversal agents) that activate coagulation by alternative pathways (3) replacement of the coagulation factors that are inhibited by the anticoagulant (4) Drug-specific antidotes that neutralize the anticoagulation of the drug [15].
However given that they were still pregnant at the time, researchers counted them as successful reversals, yielding an overall reversal rate of 48%.
Twenty five females presenting in the OPD for reversal of laparoscopic sterilisation were counselled and operated abdominally using 4x magnification with a simple technique after ruling out infection under regional anaesthesia, i.e.
A positive correlation was seen between pattern reversal VEP latencies N75 and P100 (r = 0.559, p = 0.001); N75 and N145 (r = 0.586, p = 0.001); and P100 and N145 (r = 0.536, p = 0.001).
A company has been formed called Age Reversal Therapeutics, Inc., that aims to raise $25 million from accredited investors who appreciate the enormous value this research may yield, not just in financial terms, but by radically extending healthy human lifespans.
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.
This paper investigates whether risk reversals (RR)--a measure of the skewness of exchange rate expected distribution--contain information about future returns of emerging market currencies.
In this paper we will show the advantage of improved cell reversal tolerant anodes using new test methods that are designed to show the relative differences in stability between various anode designs.