The cross used in the telescope of a level is fixed in the eye end of the instrument, and just within the focus of the eyepiece, generally 1 in. from the eye end. But this varies according to the focal length of each eyepiece. The wires are taken from the spider, and directly laid over the diaphragm, to which they are attached. Experiments have been made with other material, but the spider's web has proved the best for the purpose. The diaphragm is a ring of metal about Jin. less in diameter than that of the tube into which it is inserted. Four screws which pierce through the tube hold it in position and serve for adjustment. The ring is bevelled in its inner circumference in order to provide a clear edge. The face to which the wires are fixed is marked off for the number and position of lines wanted; then the web is stretched across in the marks made, and secured at each end by a drop of varnish.