a paper ‘On the Properties of a certain Curve derived from the Equilateral Hyperbola,’ which was followed by others in the same series, and from that time for upwards of fifty years he wrote numerous articles on mathematics, physics, astronomy, chemistry, and archæology. In 1826 he turned his attention to the chemical action of light, the results being communicated to the ‘Edinburgh Journal of Science’ and other periodicals.
On 1 Oct. 1833, when trying to sketch the scenery along the shores of the Lake of Como by the aid of Wollaston's camera lucida [see Wollaston, William Hyde], having previously tried the camera obscura for the same purpose, and wearied by many successive failures, he was led to consider whether it would be possible to make permanent the pictures which the glass lens of the camera obscura threw upon the paper. In 1802 Thomas Wedgwood [q. v.] (son of the potter) produced evanescent sun-pictures or ‘profiles by the agency of light’ upon sensitised paper, and Talbot followed up Wedgwood's line of research. After experimenting for five years he had nearly arrived at a satisfactory consummation when he learned that his results had been rivalled by Louis Jacques Mandé Daguerre. Daguerre had since 1824 been seeking to perfect the experiments of Joseph Nicéphore de Niepce of Châlon-sur-Saône, who, as early as 1824, produced permanent ‘heliotypes’ by means of glass plates coated with bitumen. Some of Niepce's ‘heliotypes’ were exhibited in London in 1827. On 7 Jan. 1839 Arago communicated to the Académie des Sciences at Paris the fact of Daguerre's successful production upon silver plates of photographic images. On 25 Jan. following Faraday briefly described Talbot's independent invention of ‘photogenic drawing’ at the Royal Institution, and on 31 Jan. Talbot communicated to the Royal Society an account of his researches, entitled ‘Some Account of the Art of Photogenic Drawing, or the process by which natural objects may be made to delineate themselves without the aid of the artist's pencil’ (Proceedings, 1839, iv. 120–1; Philosophical Mag. 1839, xiv. 196–211). Talbot's process consisted in producing the photographic image on writing-paper highly sensitised by chemical treatment. White images of the objects were formed after a long exposure upon a dark ground, these being the ‘negatives,’ from which ‘positives’ could be obtained by printing in the manner still employed.
In September 1840 Talbot greatly improved and accelerated the procedure by employing paper rendered sensitive by iodide of silver and nitrate of silver. This paper received in the first few seconds of its exposure to the light an invisible image, which could be rendered visible by treating it with a solution of gallic acid. This improved method, at first called the ‘calotype,’ and afterwards the ‘talbotype,’ was the foundation of the photography of the present day. Talbot patented it on 8 Feb. 1841, but his claim to priority of invention in regard to this phase of the development of photography directly conflicts with that of Joseph Bancroft Reade [q. v.] In 1851, after the introduction of the ‘collodion’ process of Frederick Scott Archer [q. v.], Talbot discovered a method by which instantaneous pictures could be taken, and in 1852 a method of photographic engraving. About 1854 he secured a gloss on photographic prints by means of albumen. All these inventions were patented; but in 1852, at the solicitations of the presidents of the Royal Society and the Royal Academy, he consented to throw open his discoveries, with the sole exception of ‘portrait-taking for sale to the public.’ In December 1854 he unsuccessfully endeavoured in the law courts to enforce his patent against Sylvester Laroche, whose development of negatives by the collodion process he held to infringe his rights.
The simultaneous invention of the daguerreotype and the calotype naturally created jealousies on both sides of the Channel. Talbot found an advocate in Sir David Brewster, and the ‘talbotype’ rapidly drove the ‘daguerreotype’ out of the field. Blanquart Evrard and others who perfected the invention of photography developed the ‘talbotype’ system of printing from negatives. If the French were unjust to Talbot in the early days of photography, they made amends at a later period, and at the Paris Exhibition of 1867 awarded him the great gold medal.
Talbot's name is so closely associated with the beginnings of photography that his mathematical powers have been overshadowed. In his memoir, ‘Researches in the Integral Calculus,’ published in the ‘Philosophical Transactions’ (1836, pp. 177–215, and 1837 pp. 1–18) he gave an account of his investigations upon the comparison of transcendents, which shows that he had independently been led to consider the development and generalisation of Fagnani's theorem, and was on the track that might have led him to rediscover Abel's great theorem. In 1842 he read at the British Association (Report, pp. 16–17) a paper ‘On the Improvement of the Telescope,’ and in the 41st report (1871, pp. 34–6) there is a paper ‘On a new Method of estimating the Distance of some of the
