1. Johannes Kepler gesammelte werke (Munich, 1938–), iv, 387–9.
2. Ibid., 388–9. Galileo discussed the projection of the Sun's image through a Galilean telescope in his second letter on sunspots (Le opere di Galileo Galilei. Ristampa della Edizione Nazionale (Florence, 1929–39), v, 136–7). In tracing sunspots on a paper on which the Sun's image is projected, the use of a Galilean telescope will result in a tracing in which top and bottom are inverted but left and right are correct, while the use of a Keplerian telescope will give a tracing in which left and right are inverted but top and bottom are correct. In either case the paper has to be held up to the light and the image transferred to the back of it for all the orientations to be correct. The only advantage of the Keplerian configuration is in the continuous adjustment of the telescope to keep the Sun's image on the paper: One does not have to remind oneself to do the opposite of what seems to be indicated.
3. Rosa ursina (Bracciano, 1626–30), 129v–130r.
4. Novem stellae circa Jovem (Louvain, 1643), 9.