Dioptrice, Augustae Vindelicorum, typis Davidis Franci, 1611
[BNCF: mss. post. 25a]
In the Dioptrics, Kepler suggested an eyepiece constituted by a convergent (biconvex or plano-convex) lens. The Galileian telescope, whose eyepiece consists of a divergent lens, furnishes erect images, but its field of view is very little. The Keplerian telescope furnishes upside down images, but its field of view is much larger and brighter.