Lantern slides were patented in 1850 by William and Frederick Langenheim. They began experimenting in the 1840s with the Magic Lantern to display colorful photographic images. Originallythey attempted to use daguerreotypes. Daguerreotypes are opaque, so the Langenheim brothers sought anew medium that could be projected. They took advantage of the works of Niépce de St. Victor, who discovered a process of adhering a light sensitive solution onto glass, which created a negative. The Langenheim brothers used that negative to print onto a piece of glass, producing a positive image. This image allowed for sufficient projection. Their invention was named Hyalotype. For more information on lantern slides see the Library of Congress: American Memory website, http://memory.loc.gov/ammem/collections/landscape/lanternhistory.html