During the First Vatican Council Lord Acton stayed in Rome as a private observer and sent to his friends Gladstone and Döllinger lengthy reports on the proceedings of the Council. Some of his letters to Döllinger were published in the prominent liberal newspaper Augsburger Allgemeine Zeitung under the pseudonym ‘Quirinus’. These sensational reports, which are said to have won to the Allgemeine the considerable number of ten thousand new subscribers, were published afterwards in a single volume. An English translation by H. N. Oxenham appeared in 1870 from Rivingtons. The importance of the Quirinus letters, of which Döllinger was the main contributor, lies in the fact that they were the most detailed and best informed reports on the Council's proceedings. Though journalistic in style and approach they were qualitatively far beyond average since both contributors, Acton and Döllinger, were trained historians of international respectability. Since the publication of Acton's private correspondence with Döllinger it has been possible to distinguish more clearly in the Quirinus letters between journalism and historical fact. We will deal in what follows with Lord Acton's contribution to these letters in order to reach a better understanding of his interpretation of infallibility.