For the second half of the 19th century and into the 20th, glass was used as a support for the creation of photographic negatives.  But glass supports could also be used to create positive images, too.  The earliest positive images on glass were ambrotypes, but by 1879, as the wet plate process was abandoned in favor of pre-sensitized dry plates, other forms of glass positives came on the scene.  

Lantern slides (usually about 3.25 by 4.25 inches) were positive images on glass made to be viewed in a projector or “magic lantern.” The processed image was monochromatic, but many lantern slides were colored by hand in order to give their subjects a more naturalistic appearance. Larger glass positives, viewed by transmitted daylight or lamp light employed the same process. These larger objects were usually fitted with a frame or other mounting system that permitted them to be placed near the appropriate light source. Orotones were another form of glass positive, easily identified by the gold coloring of the plate that replaced the white or clear highlights of the image.