1. This is letter 517.
2. See for the perspective frame, which was in use as long ago as the Renaissance: letter 235, n. 10.
3. There are various drawings of perspective frame designs in Van Gogh’s Nuenen sketchbook; he may have shown these to Kerssemakers. See Van der Wolk 1987, pp. 50, 53, 60, 75-76, 79.
4. See for Stengelin and Drenthe: letter 473, n. 6.
top