opaque frame of brown Manila paper, printing sixteen or more at a time,
depending upon the size of the printing frame. Many small tracings may
be similarly mounted upon a brown paper multiple frame, and may be
printed together upon a single sheet.

* * * * *




SPECTRUM GRATINGS.


At a recent meeting of the London Physical Society, Prof. Rowland, of
Baltimore, exhibited a number of his new concave gratings for giving a
diffraction spectrum. He explained the theory of their action. Gratings
can be ruled on any surface, if the lines are at a proper distance apart
and of the proper form. The best surface, however, is a cylindrical or
spherical one. The gratings are solid slabs of polished speculum metal
ruled with lines equidistant by a special machine of Prof. Rowland's
invention. An account of this machine will be published shortly. The
number of lines per inch varied in the specimens shown from 5,000 to
42,000, but higher numbers can be engraved by the cutting diamond. The
author has designed an ingenious mechanical arrangement for keeping the
photographic plates in focus. In this way photographs of great
distinctness can be obtained. Prof. Rowland exhibited some 10 inches
long, which showed the E line doubled, and the large B group very
clearly. Lines are divided by this method which have never been divided
before, and the work of photographing takes a mere fraction of the time
formerly required. A photographic plate sensitive throughout its length