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 ?
Although hamstring extensibility improvements are expected to decrease after a period of detraining (Mayorga-Vega et al.
28], who measured PEmax, sustained vowel prolongation (SVP), words per minute (WPM) in connected speech, and quality-of-life indices related to the presence of the dysarthria and dysphonia after 8 wk of EMST, followed by 4 wk of detraining.
Detraining that occurs due to sickness or injury will compromise the training adaptations and the time to regain these losses may be greater than the time it took to lose them.
Once optimal conditioning has occurred and further improvement is no longer sought after, the reversibility principle looms, and the onset of detraining occurs quickly.
The intriguing aspect of this study was the analysis of the effects of detraining for up to 2 weeks.
The BBC explained how rehabilitation and detraining of greyhounds takes place.
In athletes with heart disease, the resolution of such arrhythmias with detraining may represent a mechanism by which the risk for sudden death is reduced.
The number of passengers moving through a station during an emergency is clearly much larger than the number of passengers moving through a station during normal operation, as all of the passengers on vehicles from both platforms will be making use of the exit stairs, not just the normal entraining and detraining passengers.
Leptin levels returned to baseline during detraining.
In May 1887, the family boarded the Canadian Pacific Railway to Port Moody BC, detraining less than two weeks prior to the arrival of the first trans-continental train at the Vancouver terminus.
I remember seeing racehorses detraining on to the station platform, then being led through the streets to the racecourse, just over a mile away.