The frame _AA_ covers two slides, _C_ and _D_. These slides are moved by the screws _F_ and _G_. The wires _E_ and _B_ are stretched across the ends of the slides so as to be parallel to each other. On turning the screws _F_ and _G_ one way, these wires are carried apart; and on turning them the other way they are brought together again. Sometimes two parallel wires, _x_ and _y_, shown in the diagram at the right, are stretched across the frame at right angles to the wires _E_, _B_. We may call the wires _x_ and _y_ the _longitudinal_ wires of the micrometer, and _E_ and _B_ the _transverse_ wires. Many instruments have only one longitudinal wire, which is stretched across the middle of the focus. The longitudinal wires are just in front of the transverse wires, but do not touch them. To find the distance between any two points in the field of view with a micrometer, with a single longitudinal wire, turn the frame till the longitudinal wire passes through the two points; then set the wires _E_ and _B_ one on each point, turn one of the screws, known as the _micrometer screw_, till the two wires are brought together, and note the number of times the screw is turned. Having previously ascertained over what arc one turn of the screw will move the wire, the number of turns will enable us to find the length of the arc between the two points. The threads of the micrometer screw are cut with great accuracy; and the screw is provided with a large head, which is divided into a hundred or more equal parts. These divisions, by means of a fixed pointer, enable us to ascertain what fraction of a turn the screw has made over and above its complete revolutions. 20. _Reflecting Telescopes._--It is possible to construct mirrors of much larger size than lenses: hence reflecting telescopes have an advantage over refracting telescopes as regards size of aperture and of light-gathering power. They are, however, inferior as regards definition; and, in order to prevent flexure, it is necessary to give the speculum, or mirror, a massiveness which makes the telescope unwieldy. It is also necessary frequently to repolish the speculum; and this is an operation of great delicacy, as the slightest change in the form of the surface impairs the definition of the image. These defects have been remedied, to a certain extent, by the introduction of silver-on-glass mirrors; that is, glass mirrors covered in front with a thin coating of silver. Glass is only one-third as heavy as speculum-metal, and silver is much superior to that metal in reflecting power; and when the silver becomes tarnished, it can be removed and renewed without danger of changing the form of the glass.