Van Gogh's perspective frame
During a significant part of his career Van Gogh worked using a perspective frame, a centuries-old artistic aid. The frame could be secured to one or two supports at eye level. Van Gogh would view his subject through the frame and on his blank sheet of drawing paper or canvas would sketch the lines that corresponded to the wires and edges of the wooden frame. In this way he was able to make an accurate assessment of the depth of field and the proportions of his chosen subject and to render these correctly onto a flat surface.
Lines such as these can be seen in many of Van Gogh's Paris paintings. Sometimes they can be seen with the naked eye or under a microscope; at other times they can only be discerned with the aid of infrared reflectography.
In a letter to his brother Theo, Van Gogh explained the advantages of working with a perspective frame. This letter incorporated small-scale sketches -- scribbles -- that he made of the frame.