Evening star. [volume] (Washington, D.C.) 1854-1972, 1911-10-15
ODD PROPERTY OF METALS

init light. AS opaque as piece of tin " is
the last word by way of comparison, and
even the penetrating X ray, which will go
through many inches of wood or other gen
orally opaque substances, is stopped by
small thickness of some dense metal In
view of this fact it will probably occasion sur
prise, then, when the statement is made that
metals under certain conditions will not only
transmit light but will show interesting Opt
cal properties, which have been the occasion
of considerable study in the last few wears
This does not mean that the popular idea
is entirely wrong, of course: but merely that
1t is not universally true. 1t is indeed a fact
that metals, even in sheets as thin as tissue
paper, will not transmit detachable amount
OF light; but if this thickness can be reduced
a hundredfOld, or even thousandfold, the
sheet will be found to be somewhat trans
parent. The best example of this is common
goldleaf, which is simply pure gold beaten
out into sheets only about one two-hundred-
thousandth of an inch in thickness In such
extremely thin layers gold will transmit
light, and, while by reflection it looks yellow
as does ordinary gold, when held to the light
it shows green color; that is gold, lust as
green glass. transmits green RAYS somewhat
more readily than the other colors. Other
metals also show this property of color by
transmitted light. Silver in thin films shows
blue, and iron is sometimes brown and sorl-e-
times shows no particular color. loOl-ing
much like piece of dark smoked glass.
But the most interesting feature of metals
as regards their optical properties is not tlis
select1ve transmission, as it is called but the
fact that they convey light with a velocity
different in most cases from its normal
speed. Taking the velocity OF propagation of
light IN a vacuum as one hundred and eighty
SIX thousand miles a second, it is practi-
cally the same in ART has been found that
in silicon the speed is reduced to one-fourth
OF this amount, and in steel about two-ffths
Copper, on the other hand, transmits light
half as fast again as its normal value, gold
three times as rapidly, and silver six.
But the metal sodium is the most inter
eStlng OF all in this connection This metal
which is one OF the constituent elements of
common salt, transmits light with velocity
no less than two hundred times its usual
value, or the inconceivable speed of thirty
seven million miles a second If light trav-
eled in free space with this speed it would
take less than three seconds to reach us from
the sun, instead of eight minutes as it does
at present, and stars whose distances are
now measured in light years. or the dis
lance light will travel in whole year. would
then be spoken of as SO many light DAYS"
away instead.
Not the least interesting feature of the
matter is the way in which these extremely
thin metallic films are produced and their
thiCkneSSeS measured Save in the case of
one or two metals, such as gold, it is impos-
sible to obtain, by hammerng or rolling, suf-
ficiently thin coherent films; so in general
the metal is deposited in thin layer on a
plate of glass. This may be done chemically
with few such metals as gold, silver. and
platinum; but the most beautiful uniform
films may be produced, of any desired thick
ness and of almost any metal, by electrical
discharge in vacuum.
The thickness of such thin LAYERS of metal
is measured by direct comparison with the
only small unit of length we know with &C
curacy; that is, the wave length of light.
The process is a somewhat complicated one,
depending as it does on a principle known as
the interference" of two beams of light;
but when carried out carefully it yields sur
prisingly accurate results, considering the
minuteness of the quantities measured.
Thus, taking the wave length of yellow light
as one fifty-thousandth of an inch, we find
that an iron film which transmits perhaps a
quarter of the light is no more than tenth
of this thickness, or one decentralization
sandth of an inch. BY comparison with this
film ones much thinner still have been meas