The latent image was Daguerres experiment with Niepce's materials. This is the process that has been registered by a silver plate, but wasnt visible yet. In 1837, Daguerre discovered that a solution of table salt dissolved in hot water would stop the light-sensitive material from continuing to react. A latent image is an invisible image made by the exposure to light of a photosensitive material such as photographic film. When photographic film is developed, the area that was exposed darkens and forms a visible image.
Marien, Mary Warner. Photography a Cultural History. Pearson, 2015.