r
the reference of new lines to standards otherwise fixed. For such
standards a relative accuracy of at least one part in a million seems
now to be attainable.

Since the time of Fraunhofer many skilled mechanicians have given their
attention to the ruling of gratings. Those of Nobert were employed by A.
J. Angstrom in his celebrated researches upon wave-lengths. L. M.
Rutherfurd introduced into common use the reflection grating, finding
that speculum metal was less trying than glass to the diamond point,
upon the permanence of which so much depends. In Rowland's dividing
engine the screws were prepared by a special process devised by him, and
the resulting gratings, plane and concave, have supplied the means for
much of the best modern optical work. It would seem, however, that
further improvements are not excluded.

There are various copying processes by which it is possible to reproduce
an original ruling in more or less perfection. The earliest is that of
Quincke, who coated a glass grating with a chemical silver deposit,
subsequently thickened with copper in an electrolytic bath. The metallic
plate thus produced formed, when stripped from its support, a reflection
grating reproducing many of the characteristics of the original. It is
best to commence the electrolytic thickening in a silver acetate bath.
At the present time excellent reproductions of Rowland's speculum
gratings are on the market (Thorp, Ives, Wallace), prepared, after a
suggestion of Sir David Brewster, by coating the original with a
varnish, e.g. of celluloid. Much skill is required to secure that the
film when stripped shall remain undeformed.

A much easier method, applicable to glass originals, is that of
photographic reproduction by contact printing. In several papers dating
from 1872, Lord Rayleigh (see _Collected Papers_, i. 157, 160, 199, 504;
iv. 226) has shown that success may be attained by a variety of
processes, including bichromated gelatin and the old bitumen process,
and has investigated the effect of imperfect approximation during the
exposure between the prepared plate and the original. For many purposes
the copies, containing lines up to 10,000 to the inch, are not inferior.
It is to be desired that transparent gratings should be obtained from
first-class ruling machines. To save the diamond point it might be
possible to use something softer than ordinary glass as the material of
the plate.

9. _Talbot's Bands._--These very r