These mirrors with different shapes are anamorphoscopes. They accompany specially made, distorted drawings (anamorphoses). These drawings seem very incoherent. However, when you look at the drawing by means of the corresponding mirror, a coherent image can be observed. Anamorphosis literally means reshape. This optical illusion grew into a very popular pastime in the seventeenth century. Instructions for making such 'distorted' drawings can be found in various texts from that time.
Anamorphoscopes
1750 - 1774