Megalethoscope slides are meant to be viewed with a Megalethoscope. This large apparatus, invented in 1870 by Carlo Ponti, allows the viewer to look at an image in two ways. First, a daylight scene can be viewed by opening the doors and letting a series of mirrors direct the light onto the front of the slide. The second view is achieved by closing the doors to the scope and directing light through the back of the slide, producing a magically transformed scene.
Also check out some other slides here. Click on the “alternate view” link below each to see the transformed view.