Daguerreotype
It is said that Daguerre has found the means to collect, on a plate prepared by him, the image produced by the camera obscura, in such a way that a portrait, a landscape, or any view, projected upon this plate by the ordinary camera obscura, leaves an imprint in light and shade there, and thus presents the most perfect of all drawings ... a preparation put over this image preserves it for an indefinite time ... the physical sciences have perhaps never presented a marvel comparable to this one.
A further clue to fixing the date of invention of the process is that when the Paris correspondent of the London periodical "The Athenaeum" reported the public announcement of the daguerreotype in 1839, he mentioned that the daguerreotypes now being produced were of considerably better quality than the ones he had seen "four years earlier".
Daguerreotype
Daguerre met with Niépce and entered into correspondence with him. Niépce had invented an early internal combustion engine (the Pyréolophore) together with his brother Claude and made improvements to the velocipede, as well as experimenting with lithography and related processes. Their correspondence reveals that Niépce was at first reluctant to divulge any details of his work with photographic images. To guard against letting any secrets out before the invention had been improved, they used a numerical code for security. 15, for example, signified the tanning action of the sun on human skin ("action solaire sur les corps"); 34 – a camera obscura ("chambre noir"); 73 – sulphuric acid.