In the 19th century photographic technology was still in it's infancy. Exposure times were incredibly long which made portraiture very difficult. To avoid blurry images, subjects were literally strapped into quite rigid frames that held their bodies stiff for the multiple second exposure. For babies and infants, the solution wasn't quote as straight forward. Still needing to be rigid for their pictures photographers came up with an ingenious, if somewhat questionable solution; to conceal the carer in the background, to quiet literally have them fade into the wallpaper.