Single prism optical inversion device.
Like a camera lens, light entering our eyes produces an inverted image on the viewer’s retina. The fact that we normally see the world the right way up is the result of adaptation using the brain’s processing capacity to ‘flip’ the image.
The effect of looking through the up-side-down scope is to optically rotate the viewer’s image around the line of sight by 180 degrees. This produces an upside down image which gives us incorrect spatial information and explains why we have difficulty carrying out simple tasks when seeing this way.