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
is got by means of a mixture of eosene, iodized collodion, and bromized
collodion. Prof. Rowland and Captain Abney, R.E., are at present engaged
in preparing a new map of the whole spectrum with a focus of 18 feet.
In reply to Mr. Hilger, F.R.A.S., the author stated that if the metal is
the true speculum metal used by Lord Rosse, it would stand the effects
of climate, he thought; but if too much copper were put in, it might
not.
In reply to Mr. Warren de la Rue, Prof. Rowland said that 42,000 was the
largest number of lines he had yet required to engrave on the metal.
Prof. Guthrie read a letter from Captain Abney, pointing out that Prof.
Rowland's plates gave clearer spectra than any others; they were free
from "ghosts," caused by periodicity in the ruling, and the speculum
metal had no particular absorption.
Prof. Dewar, F.R.S., observed that Prof. Liveing and he had been engaged
for three years past in preparing a map of the ultra-violet spectrum,
which would soon be published. He considered the concave gratings to
make a new departure in the subject, and that they would have greatly
facilitated the preparation of his map.
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