Evening star., 1938-03-05
STARS, MEN AND ATOMS
Notebook of Science Progress in Field,
Laboratory and Study.
BY THOMAS r. HENRY.

Determination of more than 100 new
basic measuring sticks Of nature is an
nounced by the Bureau Of Standards.
The fundamental length" in crea-
tion recognized by physics is precisely
6,438 1896 angstrom units-one angstrom
unit being precisely .00000O003937 Of an
inch. 1t is the length of one wave Of red
light emitted by the element cadmium
when it is made luminescent. This is
length which is the same everywhere
in nature and which never will change.
The yardstick and the foot rule are man
made things. Their ends will wear away.
They will stretch and shrink Any one
of them is apt to change by billionth
of an inch or so almost any time.
But if all man-made standards of
measurement were to be destroyed they
could be rebuilt with this one wave length
of red light. BY means Of 1t any two
such standards can be precisely COM
pared. This particular unit was taken
almost half a century ago when DR..
Albert A. Nicholson made precise cali
bration of the meter stick in terms Of it.
1t happens to be sharp line in the
spectrum of cadmium which can be
produced with comparative ease.
But checks" were needed. even on
this fundamental unit. theoretically,
any wave length of radiation would serve
as such check if it appeared sharply
in the spectrum so that it could be very
accurately measured and its length de-
termined. For a long time physicists
have been searching for lines of this
sort which can be used as secondary
tandards-comparing with the almost
enthusiastically short wave length Of the
red line of cadmium as the inch COM
pared with the foot or the millimeter
with the centimeter.
One essential purpose of this is to have
measuring stick with which to measure
other wavelengths of radiation-upon
the exact determination of which de-
pends the development of spectrum
chemistry and, especially, the determina
ton OF the elements in the makeup of
the stars, planets and outer space. The
International Astronomical Union spon-
sors the specification and adoption of
these units. A prerequisite for the adop-
ton OF any radiation wave length as an
international secondary standard IS that
the length must be determined with no
margin of difference whatever by three
independent observers.
1nlS is very Slow process. Thus lar
total Of 284 such lines have been
adopted. Twenty each are emitted by
the elements krypton and neon. Iron
has furnished all the rest, 244. Every
one OF the 81 elements, when made
luminescent, gives out light precisely
Its swn. Not single wave length of
this light is duplicated by the light of
any other element A single wave length
belonging to iron in the light of star
would constitute irrefutable evidence that
the element was present in that star
providing, of course, that everybody
agreed on the Identification of the line.
But it so happens that all these 284
lines are yardsticks. They are all, with
the exception of few Of the iron lines.
within the wave-length range of visible
light, from the deep red to the deep violet.
There Is need Of some inch STICKS" pre-
cisely determined wave lengths of light
lines in the ultraviolet, where they are
SO short that they have no effect on the
human retina, and hence are invisible.
Lines are difficult to measure precisely
in this invisible region.
1t SO happened that the rare gas ele-
ments krypton, argon and neon contain
some reasonably sharp ultraviolet lines
DR.. Curtis j. Humphreys of the Bureau
Of Standards spectroscopy section has
made exact determinations Of the wave
lengths Of 104 Of these, nearly all In
the region where the wave lengths are
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