Evening star., 1937-05-08

Blue eyes are not really blue, nor red
sunsets really red.
The apparent color is due to the scat
tering of light when the photons hit
against extremely minute particles. 1t
takes place only when the diameter of
the obscuring particles is small in COM
parison to the wave length of the light.
For this reason light in the blue end
of the spectrum is scattered more than
light in the red end.
This principle, worked out more than
a half century ago by the English
physicist. Lord Rayleigh, was explained
at the annual meeting of the American
Chemical Society at Chapel Hill by Drs
D. L. Gamble and c. E. Barnett OF the
research division Of the New Jersey Zinc
Co. as a hitherto unappreciated means
of measuring particle size and distribu-
ton.
Thus" Dr. Gamble pointed out "the
sun appears red when viewed through a
thick smoke fog. but white through a
fog in the country consisting of water
droplets. Water drops are too large to
produce true scattering. Their action is
in the nature of difTraction and reflec-
ton. which is almost equally effective
for all wave lengths. Hence water drops
appear white, whether viewed by re-
flected or direct solar radiation.
"Other examples of structural colors,
which are produced by scattering phe-
nomena are the red color of the sunset,
the colors of bird feathers, the blue color
of the sky or of skinned milk. This
blue is now known as TVndall blue.
There is no blue pigment in blue eyes
and 1t was pointed out by the English
physicist Tyndall years ago that the
blue Of the eye is really the blue Of turgid
media and is analogous to the blue of the
sky or the blue of skinned milk.
During the first two or three years
after the eruption of Krakatoa a red
dish-broWn corona was often observed
around the sun. This was known as
Bishops ring. 1t was explained as the
result of the scattering of sunlight by
small dust particles, the blue being scat
tered back toward infinite space, while
the red was unaffected. Assuming that
the particles were spherical, it was pos
sible to calculate their diameter.
"If such observations are confined to
radiation in the visible spectrum, one is
able to measure only the diameters of
extremely small articles, since the wave
length of light through the visible spec
trum is small and the diameter of the
particles which will scatter such radia-
ton must be still smaller. BY extending
the observations into the infrared how
ever, the great increase in wave length
makes possible the measurement of the
diameters of coarser particles with con-
siderable accuracy. For example, If it is
desired to determine the size of pigment
particles in a paint, a small amount Of
the paint Is diluted until it is translucent
with some clear paint oil. A cell Is
filled with this suspension Of pigment in
oil and mounted In front of the entrance
slit of a spectrometer. Light from an
ordinary automobile headlight bulb,
which is rich both in visible and near
infrared radiation is focused on the cell.
That transmitted through the suspension
is separated into spectrum OF single
colors or wave lengths extending from
the visible blue to the infrared.
"A thermocouple is used to measure
the intensity of the radiation represent
ing each one of these wave lengths and
the intensity of the radiation transmitted
at each wave length is plotted against
the wave length. From the equation.
which relates the intensity Of the trans
mitted radiation with the diameter OF
the suspended particles and the wave
lensth Of light the average diameter q