The Ketchum keystone. [volume] (Ketchum, Idaho) 1885-1899, 1898-03-05
LIGHTS ON THE. OCEAN.
Interesting Experiments with the
Visibility OF Different Colors.
The 0overnm ents Of Several Nations
Deeply Interested In Ascertaining
Which Arc the Best Lights
for Signals.

bilitv of the lights at sea has been pub
lished by the naval photographic Office
as a result of special Investigations
undertaken by officers of this govern
ment, Germany and the Netherlands.
A marine observatory in Germany in
this connection tested some three thou
sand running lights actually in use on
board ships and found that two-thirds
of them failed to meet conditions pre-
scribed by the international maritime
conference held in Washington in 1889.
1t was then stipulated, says Wash
ington4etter to the Baltimore Sun, that
the word visible" should mean visible
on dark night with clear atmos-
phere. The Germans have further
pursued the inquiry by systematic
series Of experiments. A light was set
up on shore and observers embarked
on board steamer which ran over
well-buoyed course. The point at
which the light ceased to be visible
was noted by each observer independ-
emily, as we'll as the point at which it
first appeared on the return trip. The
mean of the several observations was
taken as the limit of visibility of the
light under observation. The naval
photographic Office sends out the fol-
lowing:
The American experiments under
taken at Long Beach light station gave
the following results in very clear
weather. A light of 1-candle power
was plainly visible at 1 nautical mile,
and one of 3-candle power at 2 miles. A
1o-candle power light was visible with
a binocular at miles, one of % can
dies family at and one of 3s candles
without difficulty at the same distance.
On q second evening, exceptionally
clear, white light of 3.2-candle power
could readily be distinguished at 8,
one of Sc at and one of 17.2 at miles.
The governmental experiment con-
Quoted at Amsterdam gave the follow
ing results: A light of 1-candle power
was visible at l nautical mile, 35 at q
and 16 at miles.
The German observations represent
more varied conditions and a large
number of experiments with lights of
widely-different character and inten-
sity.
In experiments with colored lights it
is only necessary to use the green, as
it has been conclusively proved that if
a light of that color fulfills the required
tests, red one of the same intensity
will more than do so. Comparisons of
the observed and computed values of
the visibilities of green lights disclose
the fact that the calculated are smaller
ohe lsc<o
and greater for long ones; in other
words, the green rays are subject to
greater absorption by the atmosphere,
even when perfectly clear, than rays Of
white light. Hence it is that instead of
being proportional to the square Of root
candle power. as in the case OF white
lights, the intensity of green lights is
proportional to the cube root of the
same, and the factor for multiplying
this quantity is, as determined by Ger
man experiments, 0.S0, the distance in
miles at which green light or single
candle power is just visible.
From this rule the candle power re-
quircd for green light to be visible at
one, two, three and four nautical miles
is two, fifteen, fifty-one and one hum
dred and six respectively. The expert
ments at Long Beach cited above gave
for green light 3.2 candle power fairly
visible at one mile, and 2S.5 clearly at
two miles, these results, however, be-
ing from a limited number Of expert
ments. The German trials were much
more numerous.
The extraordinary rapid diminution
of the visibility of the green light with
weather, and the still more rapid de-
crease in rainy weather of character
which will but slightly diminish the in
tensity of white light, show that it is
of the utmost importance to select for
the glass shade OF color which will
interfere with the intensity of the light
as little as possible. The shade recon-
mended is pale blue-green. Yellow
green and grass-green should not be
employed, as they become indistin-
guishable from white at very short
distance. For the red considerably
wider range is allowable, but cop
perry red is probably the best.
With regard to the loss which the
light suffers in being transmitted by
the glass, tests made show that this va
ries from fourteen per cent. in the case
of perfectly clean glass to fifty-one
per cent. in the case OF one in the
smoky condition due to nights burn
ing.
GRAND OAK IN SOUTH CAROLINA