Make a Pseudoscope
The pseudoscope does to front and back what a mirror does to left and right. This means that foreground becomes background and visible background becomes foreground, or more simply, background advances, foreground recedes.
So when you view objects with this scope, the convex becomes concave. So a brick lying on the ground appears to be a brick-shaped hole sunk into the ground. A tree turns inside out as it were – it’s front branches appear at the back, whilst the back branches come out in front, hanging or suspended in mid-air as their support is eclipsed by branches in front.
The scope will suggest to the viewer that a person’s face is hollow or concave but your brain refuses to accept such nonsense and so peoples’ heads merely appear odd or uncomfortable to look at. It is best then to choose to look at a landscape or tree or anything which may suggest ambiguous perspective.
