5 Bone telescopes from Amsterdam
5.1 The bone telescopesnext section
Three almost complete telescopes (fig. 2, 4, and 7) and two fragments of telescopes were found in excavations in Amsterdam. The fragments found in Amsterdam could be identified as telescopes by comparing them with the nearly complete bone telescopes (Rijkelijkhuizen 2004). Four of the telescopes consist of a single part. The length of these four telescopes is between 79.7 and 89.6 mm. The largest diameter measures approximately 24 mm. The fifth telescope is made from two parts, which are fixed together with a screw-thread. The total length of this telescope is 138.7 mm; the lengths of the individual parts are 68.9 and 76.7 mm. The largest diameter of the bone tube is about 25.5 mm.
Figure 2 Bone telescope with view of the lens (VIN9-1). Collection: Bureau Monumenten & Archeologie (BMA), Photo: Wiard Krook, BMA.
Figure 5 Marrow cavity is visible on one of the telescopes. Collection: BMA, Photo:Marloes Rijkelijkhuizen.
Figure 7 Telescope made of two parts, with view of one of the lenses and the aperture (SIN1-4). Collection: BMA, Photo: Wiard Krook, BMA.
Two of the telescopes were recovered from cesspits and could be dated to the 18th century. One was found in a cesspit in the Vinkenstraat and is dated between 1725 and 1775 (VIN9-1); the other was excavated in the Korte Houtstraat, near the Waterlooplein, and is dated between 1725 and 1750 (WLO-179-12). The other three have no clear context (A-448, SIN1-4, VARA-6-3), but the similarity in their appearance suggests that these also date to the 18th century.
5.2 Raw material and manufacture
The telescopes were made from cattle metatarsals. Cattle metatarsals are especially suitable because they are quite round and have a thick compacta. The length of the telescopes is about 8 to 9 cm, this is about the limit for making telescopes because the marrow cavity becomes more oval towards the epiphyses. After the removal of the epiphyses, the outer side of the diaphysis was modified on a lathe. The marrow cavity was only modified on each end to create a platform for the lenses to rest on. On both ends screw-thread was formed on the outer side for the lens caps. When the bone tube of the telescope was finished, the lenses were put in place with metal rings (Peter Louwman, pers.comm.).
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.