When you look through the little glass/mirror you can see the entire ship, clever work from 1744-ish…
An anamorphic painting, a composition that can only be viewed with a special mirror that restores the deformed image. This example was originally attributed to Van de Velde, a Dutch artist, but its origins are now uncertain. Painted on a wooden panel, the painting is unrecognisable until a cylindrical mirror is placed at its centre over a circular portrait of a lady. Viewed from above, a sailing ship without rigging will suddenly heave in sight. Paintings of this type were quite popular during the 18th century, especially anamorphic portraits that concealed the identity of the sitter from prying eyes.
Ingenious – Link.