[Illustration: Micrometer caliper measuring 1-1000 inch. Brown & Sharpe, Providence.] [Illustration: Plug and ring for standard measurements.] Refined Measurement Improves Machinery. In manufacturing modern tools and machinery, the thousandth of an inch is the usual limit of allowable error. A micrometer caliper measuring to this limit is here shown. The pitch of its screw is 40 to the inch, and the beveled edge of the screw-thimble is divided into 25 parts, so that motion from one division to the next takes the screw 1/25 of 1/40 of an inch, or 1/1000. By carrying refinement a step farther, 1/10,000 of an inch can be detected. The production of a screw such as this was simply impossible by the lathe as used almost up to the close of the eighteenth century, its operator holding in his hand a gouge or chisel. Of inestimable importance was Henry Maudslay’s invention of the slide-rest which firmly holds the tool, moving it automatically along the wood or metal being cut. See illustration on page 96. James Watt, as he endeavored to improve the steam engine, before the slide-rest was invented, was sorely vexed and thwarted by the ill-shaped containers for steam which served him as cylinders. Perhaps the chief task accomplished by the lathe has been its own improvement, so that to-day surfaces are readily cut by its tools accurately to within a thousandth part of an inch. Vastly beyond this feat was Professor H. A. Rowland’s production of a virtually perfect screw, which enabled him to rule on concave gratings 5.9 inches square, 110,000 lines with such precision that the error between any two of the lines is probably less than 1/3,000,000 of an inch. These gratings brought to view spectra much more extended and clear than those observable in a spectroscope, however powerful. The concave plates employed by Professor Rowland were made by Mr. John A. Brashear of Allegheny, Pennsylvania. Measurement is greatly indebted to accurate means of enlarging the images of objects as viewed in the telescope or the microscope. Glass grinding tools are to-day so exquisitely contoured that a lens forty-two inches in breadth shows the image of a star as an immeasurable dot. It was in pressing together two lenses of very large and known radius that Newton measured the lengths of light-waves. With homogeneous rays, such as those of yellow light, the successive rings of light and darkness marked the points at which the intervals between his lenses were equal to half a light-vibration or any multiple thereof. Measuring these intervals, by noting their distances from the common centre of his lenses, he found the wave-length of the particular light he was studying. [Illustration: Two lenses as pressed together by Newton.] [Illustration: Newton’s rings as produced in yellow light.] Interchangeability Old and New.