[The original has not been located. This is the extract published in Talbot's letter to the editor of the Literary Gazette, no. 1277, 10 July 1841, p. 445. Talbot's full letter is Doc. No: 08589.]
[Munich, June 28, 1841]
You are, perhaps, aware that Professor Petzval in Vienna, has invented a new combination of achromatic lenses, in order to make the whole aperture useful. Each of the achromatic lenses has an aperture of one and a half inch diameter, and both act together with their full apertures as a single achromatic lens of five and a half inches focus. The diameter of the image in its greatest distinctness is more than four inches. Professor Petzval's first experiments with this instrument were made in the autumn of last year. The plate was prepared with tincture of iodine, according to the process of M. Asherson of Berlin. On a clear day, the person sitting in the shade, never more than two minutes were required to obtain a perfect portrait. The present method of preparing the plate, is, however, somewhat different. Iodine is exposed to the vapours of chlorine, and then the watery solution of the resulting brown semi-fluid combination is used instead of the tincture of iodine. Under these circumstances a portrait never requires more than eight or ten seconds, and whole groups of figures obtained in this way make a particularly beautiful effect. According to Professor Von Ettingshausen, the plates become still more sensible when they are first iodised in the usual way, and then held over a feeble solution of chlorine in water till they have assumed a slight reddish tint. Such a plate requires only two seconds in shade, and less than one second in sunshine, but the result is not certain. The brothers Netterer of Vienna made another interesting discovery. They found that a plate treated as usual with iodine, and afterwards exposed in the dark for a short time to the vapours of dichloride of sulphur, received in one minute, in the camera obscura, no perceptible image. But it appeared immediately when the plate was slightly heated, or exposed to the light.