The Sabatier effect, also known as pseudo-solarization (or pseudo-solarisation) and erroneously referred to as the Sabattier effect, is a phenomenon in photography in which the image recorded on a negative or on a photographic print is wholly or partially reversed in tone. Dark areas appear light or light areas appear dark. Solarization and pseudo-solarization are quite distinct effects. Over time, the "pseudo" has been dropped in many photographic darkroom circles and discussions, but the effect that is meant is the Sabattier effect and not the solarization by extreme overexposure (see below). Background Initially, the term "solarization" was used to describe the effect observed in cases of extreme overexposure of the photographic film or plate in the camera. The effect generated in the dark room was then called pseudo-solarization. Spencer defines the Sabattier effect as: "Partial image reversal produced by brief exposure to white light of a partly developed silver halide image". Many other ways of chemical and actinic radiation "exposure" can be utilised for the partial image reversal. The use of chemicals for image reversal is also known as 'chemical fogging'. The SPSE Handbook of Photographic Science and Engineering describes the effect as follows: If a film that has been exposed, developed, and washed but not fixed is given a second uniform exposure and developed again, an image with strong border effects is obtained, which combines the original image with a reversed (positive) image. Another usable definition is by Wijnekus & Wijnekus: If an exposed, incompletely developed, and washed, but not fixed film is given a second uniform exposure and developed again, a reversal of the original image may be obtained. The reversal may be partial or complete, depending on the relative magnitude of the first and second exposures. History The Pseudo-solarization effect was described in print by H. de la Blanchère in 1859 in L’Art du Photographe. It was described again in 1860 by L.M. Rutherford and C.A. Seely, separately, in successive issues of The American Journal of Photography, and in the same year by Count Schouwaloff in the French publication Cosmos. French scientist Armand Sabatier published 26 October 1860 a process of obtaining direct positives (referencing Count Schouwaloff and Poitevin), but according to the description, this process did not seem to have any connection with the Sabattier effect as no mentioning was made of any exposure of the collodion plates after development had started. The name of the