_The Ramsden Eye-piece._--The cobweb micrometer is the most efficient piece of apparatus yet brought into use for measuring the magnified image. It is made by stretching across the field of the eye-piece two extremely fine parallel wires or cobwebs, one or both of which can be separated by the action of a micrometer screw, the trap head of which is divided into a hundred or more equal parts, which successively pass by an index as the milled head is turned, shown in Fig. 104. A portion of the field of view is cut off at right angles to the filaments by a scale formed of a thin plate of brass having notches at its edges, the distances between which correspond to the threads of the screw, every fifth notch (as in the previous case) being made deeper than the rest, to make the work of enumeration easier. The number of entire divisions on the scale shows then how many complete turns of the screw have been made in the separation of the wires, while the number of index points on the milled head shows the value to the fraction of a turn, that may have been made in addition. A screw with one hundred threads to the inch is that usually employed; this gives to each division in the scale in the eye-piece the value of 1/100th of an inch. The edge of the milled head is also divided into the same number of parts. [Illustration: Fig. 104.--Ramsden Screw Micrometer Eye-piece.] In Watson’s Ramsden screw micrometer, Fig. 104, the micrometer scale (seen detached) is ruled on a circular piece of glass, and this, by unscrewing the top, is dropped into its place, and one of the wires, both being fixed, is set a little to the side of the field, the teeth of the screw being cut to 1/100ths, and the drum giving the fractional space between the teeth to 1/100ths, so that the 1/10000th of an inch can be read off. This micrometer eye-piece is constructed entirely of aluminium, a decided advantage, being so much lighter than brass to handle. In the screw micrometer of other makers, other modifications are found. An iris diaphragm being placed below the web to suit the power of the eye-piece employed, a guiding line at right angles to the web is sometimes added. Care should be taken to see that when the movable web coincides exactly with the fixed web, the indicator on the graduated head stands at zero. _The Compensating Eye-piece._--The very important improvements effected in the construction of the objective naturally led up to an equally useful change for the better in the eye-piece.