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
In the Becquerel variation of the process, published in 1840 but very seldom used in the 19th century, the plate, sensitized by fuming with iodine alone, was developed by overall exposure to sunlight passing through yellow or red glass. The silver iodide in its unexposed condition was insensitive to the red end of the visible spectrum of light and was unaffected, but the latent image created in the camera by the blue, violet and ultraviolet rays color-sensitized each point on the plate proportionally, so that this color-filtered "sunbath" intensified it to full visibility, as if the plate had been exposed in the camera for hours or days to produce a visible image without development.