For centuries the mirror, or its equivalent piece of polished metal, has been used by artists to help them see the shapes and values of the image they are painting on a flat plane in reverse.
Indeed Leonardo da Vinci in his treatise on painting entitles ones section
“Come lo specchio é maestro de’pittori” The mirror is master of painters
There are so many advantages to using the mirror. If you hold it perpendicular to your strong eye and look in reverse at both your artwork and subject you are painting, it gives you a completely fresh view- retrieving the innocent eye. it also eliminates everything in your periphery. The flat plane of the mirror imitates the flat plane of your canvas, panel or paper.
In the book “Painting a Portrait by De Laszlo”, which Philip de Laszlo produced with his friend and art critic Alfred Lys Baldry he says that the mirror’s “chief value is that it gives me a new vision of both picture and sitter and therefor enables me to discover any faults there may be in drawing, or in the relations 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.
The Black mirror has the added advantage of reducing the value range. I use my iPhone as a handy equivalent, though it can give you quite a shock when it rings mid concentration. We have recently got black mirrors in stock at the school shop Lavender Hill Colours.