Lantern slides are sandwiches of glass plates, approximately 3 inches by 4 inches, with photographic images. The early slides have pictures printed directly onto the glass. Later slides have photographic film held between the pieces of glass. They were originally viewed in much the same way current 35mm film slides are viewed, with a projector developed for that purpose. However, the light for the projection originated from a lantern, not an electric bulb, although oil and gas lanterns were eventually replaced with electric ‘lanterns’.