, 1916-02-19

OIL UQG DElIlSu INSTRUMENTS Nt1G PulNA1ttoNo
before the war and are III use by the enc
my; their extraordinary perfection has
caused them to be used almost exclusively
in the French Army too. The Turkish
Army are using British feld.g@lasses in
very large numbers. Yet until the war
and even up to the preseut-the idea
was very prevalent among the British that
what are known as 'precision instruments"
in the optical way must be of German orr
gin if they are really of any use. A great
new industry has been established i'll our
country through the war, and already Bri-
tish opticians have established the fact that
they stand pre-eminent in a vast branch of
manufacture hitherto allied in popular Opt
Mon as much with Germany as is the ami
line dye industry.
There have been difficulties to contend
with which cannot be laid to the blame
of any lack of foresight in this country,
such as the scarcity of certain types Of
glass. A very dense type of lin gass,
for example, is used in the manufacture of
prisms; locality plays an important part
in the nature of the raw material, and
most of this glass came from Jena. 1t is
being made both in France and England
in increasing quantities, but in the mean
time our soldiers have had to use trench
periscopes made for only one eye in order
to conserve the supply Of prismatic glasses
whereas the German periscopes are blno-
cular.
1t may be surprising to learn what an
important part the prism plays in means
of sighting in warfare. Trench periscopes,
rifle and other telescope sights, prism
binoculars, range finders, nearly every
type of instrument, takes advantage of
the reflection without appreciable loss of
light which a prism, as distinguished
from an ordinary mirror, will give.
The use of a prism also enables the
human eye to be placed at a lower level
than the 'eye" Of the instrument which
means a great saving of human life. The
now almost universal telescope rifle sights
not only bring the enemy visibly much
nearer, but the soldier is able with them
to see what he could not see with the
naked eye unless he held his head an inch
or two higher above the trench.
RANGE FINDERS.
The men working a gun to day are very
much like the engineers in a ship: obeying
commands without seeing where they are
going ar what exactly they are doing.
The range of the enemy is found by means
of a range finder. Simply described, this
instrument comprises two telescopes about
three feet apart, one of which throws an
erect image on an eyepiece near the mid
die of the instrument; the other an inver-
ted image. A delicate adjusting screw is
turned until the erect image comes exactly
above the '|upside down" image, when the
range can be read off up to ten or fifteen
thousand yards on a scale. The gun is
trained, following on these details, by artil-
lery officers, allowance being made for
the continual falling of the shell during
it's fight through the air, and the men
who actually fire the guns are mere automa-
tons, acting as the mechanics who feed
and drive the engine Of destruction.
British opticians, as already stated,
stand p~re-eminent in the making of range
finders, although a German instrument is
included in the official artillery textbook
and was used by our Army for many years.
Much might be written of British in
ventions in the science of sight which
seems to border on the miraculous. There
is a little instrument known as the corner
cube prism which, if fitted to a vessel at
sea, makes that vessel distinguishable
from any other by those on shore who
have the necessary sighting apparatus,
though the ship may be miles away. This
little instrument, hardly bigger than a
lump of sugar, possesses the remarkable