5.3 The optical system
One of the lenses is still present in one of the telescopes (fig. 2). Both lenses are preserved in this telescope that consists of two parts (fig. 7). The telescopes were therefore refracting telescopes. But which optical system was used, the Dutch/Galilean or the Keplerian/astronomical system? Whether the lenses in the telescopes are convex or concave lenses cannot be seen. The small telescopes found in Amsterdam probably had a low magnification. Following the rapid development of the telescope soon after its invention, larger and better telescopes were available in the 18th century. These small telescopes were probably personal items for terrestrial use. A Dutch system is more convenient for telescopes for terrestrial use because of the upright view. Keplerian telescopes could also have an upright view when an extra third lens was installed in the telescope. The telescopes found in Amsterdam only have two lenses. It can therefore be concluded that the telescopes used the Dutch system.
Simple lenses can suffer from spherical aberration, caused by the light passing through the edges and the centre of the lens coming to focus at different points. The size of the objective lens limits the amount of light that comes into the telescope (Pedrotti & Pedrotii 1993: 109-110). The diameter of the lens could be reduced to lessen spherical aberration (Allen 1943: 304). The effective aperture of the lens is already limited by the size of the opening against which it is fixed. It is also possible to further reduce the aperture of the lens by means of a lens cap with a small opening. It is unknown whether the lens caps were for protection of the lenses only or also to reduce the effective size of the lens.
Another influence on the amount of light could be the inside of the bone tube; because the marrow cavity is not enlarged, the thicker bone around the middle of the tube might block the outer rays of light (fig. 6). In the case of the telescope made from two parts, the marrow cavity is reduced further by putting in a bone insertion with a small opening (fig. 7). This functions as an aperture stop (Van Helden, pers. comm.). It blocks the outer rays of light, which results in a sharper view by reducing the spherical aberration.