They include daguerreotypes, ambrotypes (which use the wet plate collodion process), tintypes and albumen prints. Oil paint was often applied to tintypes, daguerreotypes, and ambrotypes. In the late 1840s, Mayall stressed the artistic qualities of his daguerreotypes. Two very important daguerreotypes (an early photographic process) were purchased in the 1990s. Sales of conventional uncoloured and hand-coloured daguerreotypes fell in anticipation of this new technology. Stereoscopy was invented in 1838 and became extremely popular for daguerreotypes, including the erotic images. Although daguerreotypes are unique images, they could be copied by re-daguerreotyping the original. He made his first daguerreotypes by 1847. Early daguerreotypes of Poe continue to arouse great interest among literary historians. It is one of only four daguerreotypes of Douglass known to exist. The daguerreotypes were not without drawbacks, however. Mayall stressed the artistic qualities of his daguerreotypes. He began making daguerreotypes through a microscope. However, because daguerreotypes only produce a direct positive print, no duplicates can be made. Despite this drawback, millions of daguerreotypes were produced.