Unconfirmed theory and research that Rembrand painted his portraits using mirror projections.
Some 17th-century artists used a system of concave and flat mirrors to achieve more realistic self-portraits. Perhaps such famous painters as Rembrandt and van Rijn resorted to this technique. It is worth noting that all the necessary tools were available to the masters of the time.
To create the projection of the artist needed daylight, as well as a concave and flat mirror. Concave mirrors, by the way, have been known since ancient times. There is a legend that Archimedes burned the Roman fleet with them,
The earliest self-portraits of Rembrandt hit the level of detail, which is achieved even in small images. A number of these works were etched on copper. The etched images had a small scale, which was happening, according to the researchers, because when using a projection, an increase in the reflected portrait led to blurred edges, making work with a sharp pen more difficult.