The mirror., 1916-02-10
TRAY NOTES..
A Delicate Measuring Instrument

Delicate, accurate instruments are required by
the physicist in making the measurements by which
the fundamental quantities of time, mass and length
are determined. The accurate determination of
length or distance has long been one of the most
difficult problems with which the physicist and
astronomers have had to deal. For long time
the most delicate instrument for this purpose was
the microscope but it's use was limited by the fact
that it's operation is dependent on light and light
waves have definite, limited length. The micro
scope is useless for studying objects smaller than
half wave length of light. Sunlit objects small
er than 1-8O, 900 of an inch cannot be seen under
the microscope and if objects under its length are
closer together than 18O 900 of an inch apart they
appear
Stimulated by the need of an instrument more
powerful than the compound microscope, Profs,
Nicholson and Morley, physicists of Cleveland, o.,
invented what is called an interferometer," con-
sistng principally of a system of optical mirrors
so arranged to divide and reflect waves of light
a certain number of times, thus making it posi-
BILE to study objects 10 times smaller than could be
viewed with the best compound microscope. This
instrument valuable aid to SCIENTISTS open
ing up to them interesting fields that had previous
ly been explored only partially or not at all, but
like the microscope it has it limitations and these
precluded a vast number of important problems
Thanks to Prof G. w Chamberlain, physic-
ist now president of Denson university, Gran
vile, O., scientist now have an instrument so del
lcate and exact that distances small 12O OOO-
900 of an inch can be measured From an article
in the Scientific American some time ago we gath
er the following information concerning this val-
able instrument, called compound interferometer:
1t is 40 times powerful the most perfect
microscope ever made. In order that the extreme
delicacy of the compound interferometer may be
adequately understood the following comparisons
given 12O, 900, 900 of an inch, the infntesmal
distance measurable with it, is the apparant size
of ordinary tin viewed at distance of 227 miles;
the size of silver dollar viewed at distance of
5000 miles or of human face viewed at distance
equal to twice the circumference of the EARTH
The making of mirrors for this instrument
was an extremely delicate task but so we'll and ac-
curately was it done that they nowhere out of
true more than IL 900, 900 of an inch. The ways
on which the carriages of the instrument travel
have to be flat and parallel; those of the first in
strument accurate that if prolonged to
distance of two miles, there would not have been
over 116 of an inch variation in their parallelism
Light waves entering the instrument from the
source of light split up and refec Jed number
of times by means of system of accurately made.
perfectly fiat, parallel mirrors and the occurrences
in the instrument observed by means of
telescope. A set of yellow and black fringes-
interference fringes- seen. With the instru-
ment adjusted to it's lowest power the distance be-
tween the two fringes represents one half wave
length of light. By changing the angle between
the mirrors, making it smaller so that the light
will be reflected otener-that is, by increasing the
length of the path of lght-the sensitiveness of
the instrument is increased to the maximum power,
namely, 40 times that of the Cleveland scientists
nterferometer and 400 times that of the best CON-
pound microscope.
In illustration a heavy steel plate with
two inch post side, all forged out of solid
piece of steel and weighing $50 pounds, is shown
so arranged as to demonstrate the compound inter-
ferometer's senstiveness. The steel post which is
capable of transmitting 200 horsepower would or
dinarily be considered quite rigid. The instru-
ment, however shows that this post actually bends
when someone merely touches it as lightly as posi-
BILE with the finger. If the breath is blown slight
ly against the post distinct movement is indicated
by the instrument and if a match is dropped on the
plate the post will be observed to futter like
feather in the wind. Even when touched lightly
with cameFs hair brush the resulting movement
of the post be detected with the interferomter
All these movements are detected with the instru.
ment set at only 11O of it's real sensibility
iuder
Compensation