Reversal

(redirected from reversals)
Also found in: Dictionary, Thesaurus, Medical, Legal, Financial.

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 ?
TABLE 2 Initial margin requirements and stock price reversals, 1902-87 Percentage of observations in which stock index reversed Standard Initial margin deviation (percent) Observations of return E([R.
Negative evidence must be evaluated when looking at reversals of existing taxable temporary differences and, if necessary, future taxable income other than reversals.
The big difference in the exposure draft is in how the future tax effects of reversals are measured.
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.
During repeated reversal training, two to eight reversals were conducted, according to the participant's performance on the 20-trial sessions.
But the truth is that, when completed carefully and after a thorough risk assessment, pipeline reversals can be safe and effective.
Additionally, in connection with the reversal of the accounting for the Uncertain Tax Position Reserve liability as previously recorded, the Company expects that based on federal income tax returns as filed, an additional USD24.
The science shows that while the conditions that cause polarity reversals are not entirely predictable, the north pole's movement could subtly change direction, for instance - there is nothing in the millions of years of geologic record to suggest that any of the 2012 doomsday scenarios connected to a pole reversal should be taken seriously.
With this letter we are informing our colleagues that the laparoscopic alternative for reversal could be offered to patients when experienced endoscopic surgeons are available.
Subjects that were re-anticoagulated with 100 U/kg of heparin followed by a second dose of PMX-60056, showed a consistent response time to the initial heparin reversal.
lt;strong>Bearish Reversal Patterns:</strong>
Here we refer to the combination of stage 1 and stage 2 as a "post-pump" reversal because these actions occurred immediately after a pump or immediately after another post-pump reversal, and are therefore distinct from the pre-pump reversals described above (see Video C, http://www.