Gratings

Gratings, in experimental Optics, are plates of gleiss or speculum metal on which are ruled parallel lines equidistant from each other, and sufficiently close to form diffraction spectra of the light that passes through or is reflected from the grating. By means of such diffraction gratings the wave-lengths of light of different colours have been most accurately determined. For ordinary purposes 10,000 lines to the inch will suffice, but for more accurate ,work double that number are desirable. The process of ruling these lines was well elaborated by Nobert, but has been much improved by Professor Rowland, who has succeeded in ruling over 40,000 lines to the inch. The process of ruling depends upon the action of a well-made screw, which takes months to make. Rotation of this produces motion of a cutting diamond, which is thus moved onwards slightly, after cutting each furrow in the glass or metal. The grating may be spoilt by wearing down of the cutting point, which will then produce more than one furrow at a time. The lines will be scratchy if the point is too hard, and the avoidance of such' practical difficulties as these may render a search after suitable diamonds one of months' duration. It takes five days and nights to rule a six-inch grating with 20,000 lines to the inch. Fairly good gratings may be produced by photography, which readily provides the means of obtaining diminished copies on glass of equidistant parallel lines.