Two plane mirrors with a goniometer. The number of the images of an oblect placed between the mirrors depends on the angle θ they format and equals (360/θ)-1 (θ in degrees).

Examples:
If θ=180ο there is 1 image
If θ=120ο there are 2 images
If θ=90ο there are 3 images
If θ=72ο there are 4 images
If θ=60ο there are 5 images
If θ=45ο there are 7 images
If θ=30ο there are 11 images