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THE instrument here described has been in use for some time on a home-made spectrometer and has given good satisfaction. Although it is not intended to replace a first class filar micrometer, its simple construction renders it a desirable piece of apparatus. As in nearly all devices of this sort, the most important part is the screw. A metal rod, preferably of brass, 2% inches long and three-sixteenths inch in diameter is threaded at one end for two inches of its length with either fifty or one hundred threads to the inch, according to the sensitiveness desired. A metal drum, with its perimeter divided into twenty equal parts by a scale, is soldered to the unthreaded end of the rod, as shown in Fig. 1. This completes the move-able part of the apparatus. A block of hard wood should be squared up to the dimensions shown in the sketch. A three-quarter-inch A micrometer of simple type. hole is bored through the fiat side of the block, at its middle point, and a four-sixteenth-inch hole is drilled from end to end intersecting the former, as illustrated. The latter hole should be bushed at each end with brass tubes, in which the threaded rod turns easily, but without play. A metal plate covers one end of the hole and a closely fitting steel pin is inserted, of such a length that the threaded rod having been introduced, the drum will be stopped about one-fourth of an inch from the end of the block. A spring presses on the drum and holds the screw firmly against the pin. A sheet metal diaphragm with a three-eighths inch aperture is fitted in a slanting position, shown in Fig. 2, so that its upper part will fall into the same focal plane as the top of the screw. A notch is cut at one side of the aperture to serve as a reference point (see Fig. 3). Brass lens tubes are attached by means of flanges and wood screws. The inside of the tubes and the diaphragm should be blackened. A positive eye-piece of one-inch focus is used. However, a single convex lens of the same focus may be substituted. After assembling the instrument, the eye-piece should be focused sharply on the top of the screw. The field will now appear traversed by a series of notches, as in- Fig. 3. When the drum is turned through a complete rotation, each notch is seen to' shift its position by its own width. Hence, fractional parts of a notch will be indicated by the drum divisions. There is obviously no lost motion and if the threads are accurately and sharply cut very precise measurements may be made. This micrometer may also be used with a microscope.
