How was Darnay granted an acquittal?

Charles Darnay is on trial for treason. Specifically, the prosecution maintains that Charles has been engaged in treasonous correspondence with the French. This is about the most serious offense that anyone in Great Britain can be accused of; it is a capital offense that is punishable by death. For all its faults, however, the English criminal justice system is a good deal fairer than the one operating on the other side of the Channel. Unlike the tribunals of Revolutionary France, English courts of law insist on the presentation of evidence before convictions are secured.
Charles's bacon is well and truly saved by his astonishing physical resemblance to Sydney Carton, junior associate of the lead defense counsel Mr. Stryver. While cross-examining a key prosecution witness, Stryver draws the witnesses's attention to the resemblance between the two men. As a good defense attorney, Stryver has introduced more than a hint of reasonable doubt into the minds of the jury, and much to Charles's relief, it proves sufficient to secure his acquittal.