This type of optical viewer, known as a zograscope or Diagonal Mirror, was popular from the mid 18th century through to the mid 19th century. The large viewing lens allowed both eyes to be used when viewing the print, this helped give an illusion of depth to the print. The angled mirror allows the print, known as a perspective view or vue d'optique, to be laid on a table behind the viewer. Prints intended for the zograscope generally show architectural subjects with strong perspective, lettering on the print is mostly laterally reversed. The example shown is typical of this type of viewer.