The morning news. [volume] (Savannah, Ga.) 1887-1900, 1897-03-14
A WONDERFUL MACHINE.
1t Engraves a Hundred Thousand
Lines to the Inch.
From the New York Tribune.

From the New York Tribune.
The announcement was made few days
ago that N large "diffraction grating
spectroseope had just been finished at the
shops of John A. Brashear, in Allegheny,
Pa, for German astronomer. This
news calls fresh attention to unique, al
most unknown and remarkable piece of
machinery, the ruling engine of Prof. Hen
~ A. Rowland, of Johns Hopkins Uni
versity, wherewith an important part of
the instrument just mentioned was made.
The object of a spectroscope is to analyze
light from a star. nebula, the sun or some
other luminous object In this operation
the image of a narrow slit is expanded side
ways until 1t is converted into long pris-
matte band or spectrum, violet at one end
and red at the other. For studying most
of the heavenly bodies, glass wedges or
prisms are employed TO perform the work.
These, properly arranged, will render good
service, where high dispersion, or mag
Indication, is not desirable. But with the
sun, whose light is so intense that it will
stand great dilution $50 to speak,, a far
more considerable dispersion is entirely
feasible. And the astronomical spectre
in order more completely to separate cross
lines in the spectrum, which are ordinarily
too close together to be properly distin-
guished. The whole point of certain re-
searches depends Altogether on the posi-
bilitiy of Identifying a suspected line pos
itively, and of measuring its position in the
spectrum with precision. Such separation
can be much better obtained with graf
ing than with prism.
There are two kinds of gratings. One
consists of glass, on whose surface a Sc
ries of fine, parallel lines are ruled. The
other Is a highly polished metal plate, sim
ilarly engraved The former lets the light
shine through It; the latter reflects II. In
either case, the effect is to produce a spec
trum exactly as a prism does. To the
metal plate a slight concavity is previ-
ously impaired, so that IL will focus the
image without the use of an extra lens
II is this latter type of grating which was
introduced into the new spectroscope.
Brashear made the mirror, gave It Just
enough curvature to make the focal dis
lance some twenty-odd feet. and then sent
IL to Baltimore for treatment in Prof
Rowland's laboratory.
Il is stated no doubt with truth, that the
lines on this particular grating are so fine
and so close together that they are 10000
to the inchl A simple comparison will en-
able one easily to realize what this means
In the majority of books there are from
300 to $50 leaves to the inch. when the
covers are pressed firmly together. In a
small minority the number will range
from 800 20 $50. These latter figures rep
resent rather thin paper Now, If we
could split one of the very thnnest of
these leaves into 300 layers OF uniform
thickness. the product would be equiva-
lent 20 10000 10 the inch, or a little less
than the number OF lines in this spectro-
scope grating. IL must be remembered,
however. that there must be "a little space
between the lines, II one is to be distinct
from the other. Therefore. II an inch be
divided up into 100000 equal parts. nine OF
them would be covered by a line and a
space. IL Is stated in one Of the news
dispatches that the space was 5000. 0OOths
wide; from which u must be inferred that
the line was 5000, 0O0ths OF an inch widel
OF course. Il is impossible TO see such lines
with the naked eye. Only a very power