(French), “A lighted chamber” (A darkened court). Before the Revolution, certain offences in France were tried in a court from which daylight was excluded, and the only light admitted was by torches. These inquisitorial courts were devised by Cardinal Lorraine. The first was held in the reign of Francois I., for trying heretics. Brinvilliers and his associates were tried in a darkened court in 1680. Another was held in 1716, during the regency. When judges were ashamed to be seen, prisoners could not expect much leniency.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894
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