For centuries the mirror, or its equivalent piece of polished metal, has been used by artists to help see the shapes and values of the image they are painting.
In his treatise on painting, Leonardo da Vinci entitles one section “Come lo specchio é maestro de’pittori” The mirror is master of painters.
The Black Mirror has the added advantage of reducing the value range.
As Philip de Laszlo writes; the mirror’s “chief value is that it gives me a new vision of both picture and sitter and therefore enables me to discover any faults there may be in drawing, or in the relation of tones. It acts like the fresh eye, which can often perceive defects that the painter, having got accustomed to them, has failed to detect. The mirror is an honest critic.”