A patent was taken out a few years ago by Mr. Barton, of the Mint, for a very ingenious method of ornamenting steel and other metals with the prismatic colours. It is effected by engraving with an engine lines on the surface of the metals of extreme minuteness, so as to divide a lineal inch into from 2000 to 10,000 equal parts. Mr. Barton has sometimes proved the correctness and stability of his engine, when drawing lines 2000 in an inch, by leaving out one line intentionally, then taking the machine to pieces; afterwards, on putting it together again, he has introduced the omitted line in its place, without causing it to be distinguishable from the rest.
In applying the principle of striated colours to ornament steel, the effect or pattern is produced upon the polished surface by the point of a diamond; so that either the whole, or a part of the surface, is covered with lines or grooves, whose distance may vary from the one-thousandth to the ten-thousandth of an inch. When these lines are most distant, the prismatic images of any luminous body, seen by reflection from the polished surface, are nearest one another, and the common colourless image; and when the lines are least distant, the coloured images are farthest from one another, and the colours are most vivid. In daylight the colours produced by these minute grooves are scarcely distinguished, unless at the boundary between a dark and a luminous object. In sharp lights, however, and particularly in that of the sun, the colours shine with extraordinary brilliancy, and the play of tints which accompany every luminous image can only be equalled by their matchless exhibition in the reflections of the diamond.
Steel dyes are engraved in this manner, from which impressions are afterwards taken for the fabrication of various articles, in which the design of the original is very faithfully transferred.