Fig. 2. Photographs of the machine. (A) The machine in working configuration, screen lowered. w, one of two wheels preventing the sphere from rotation about the vertical axis. (In the lower left corner the small sister of the 50 cm sphere can be seen, designed for insect work.) (B) Machine with screen lifted. b, DDV 1800 (Liesegang) beamer; c, two small video `finger' cameras to control the rat's behaviour within the screen; md, motion detector. In the background the air support tube and the crank handle to lift and lower the screen are visible. The photo is a portrait of the late Sigmund Exner, 1846–1926 (physiologist at Vienna). (C) The optics in more detail. aam, angular amplification mirror; p, two plane mirrors. The beamer and its shielded output is seen on top. (D) The haltering of the rat. aie, angular incremental encoder. The light gray leather harness attached to aluminum sheets with axial joints is seen. The thin brass tube ending at the rat's mouth is always rotated with the haltering and delivers sugar water drops via a PC-controlled valve. For details, see text.