Democratic messenger. [volume] (Snow Hill, Md.) 1869-1973, 1910-02-19
LIGHT AND THE EYES
We Do Not Always See Things
as They Really Are.
CoL0RS WE CANNOT DISCERN
Normal Vision 10 Not Able 10 Pene-
trste to the Regions Beyond the Vic.
Iet-soms Results That Have Been
Attained With the Camera.

Ies-Some Results That Have Been
Attained With the Camera.
Those of us who are fortunate
enough to possess normal eyes are apt
as they are This Is mistake. The
appearance of the world at large Is
merely the result Of the circumstance
that the human eye perceIves only a
comparatively small part Of the total
radiation which comes from the sun
or Is given out by lamp
To make this little clearer we may
consider one Of the oldest experiments
in optics. the passage OF ray OF light
through prism.
When beam OF sunlight passes
through q prism Of glass such as is
often found on old fashioned chande
llers the rays are spread out. forming
spectrum. which we sec as a rain
bow colored baud upon the wall. The
colors Of the spectrum merge into one
another by insensible gradations.
though the older physicists were In
the habit Of recognizing seven--red.
orange. yellow. green. blue. indigo and
violet
These colors which the eye pcrceives
are by no means the only ones present
In the spectrum. II we perform the
experiment In a dark room and place
a Photographic plnte In such position
as 10 receive the image OF the spec
trum we find that II extends to a con-
siderable distance beyond the violet.
In other words. there Is a region be-
yond the violet which the eye cannot
see. but which the sensitive plate is
able to photograph. This region Is
termed the ultra violet. and the light
from virtually all sources contains a
greater or less amount OF this ultra
violet light.
The only difference between this Ill
ira violet light and ordinary visible
light Is In the length Of the wave.
which is. In fact. the only difference
between the different colors OF the
visible spectrum II, instead OF using
glass prism for forming our spec
trum. we use one cut from a quartz
crystal we find that the ultra violet
spectrum Is very much longer. which
shows us that the shortest waves OF
light are not transmitted even by the
clearest glass.
II your eyes were sensitive to this
region OF the spectrum instead OF 10
the other the appearance Of things
would appear black. our windows
would be as Opaque rs sheets OF iron.
and polished silver would resemble
anthracite coal. In other words things
appear as they do merely from the cir
cumstance that In the process OF evo.
luton our eyes have developed sensl-
bility TO certain region Of the total
spectrum OF the light which comes Ia
us from the sun
II we have used an ordinary photo
graphic plate In our experiment we
shall find that II has received no i'm
preston from the red and yellow parts
OF the spectrum and very little from
the green. The plate therefore suffers
from the same defect as our eye. II
Is sensitive only TO limited region.
| Recently. however. means have been
found OF rendering plntes sensitive to
the entire visible spectrum and 10
certain extent TO the mira red or the
extension beyond the red analogous IT,
the one beyond the violet.
With a Photographic plate rendered
sensitive 10 the entire spectrum we
could obtain an idea OF how things
would appear II our eyes were sensI
tive 10 regions other than the on,
adopted by nature by excluding from
the lens OF the camera all rays other
than the ones with which we wish a.
operate This Is done 10 n limited eX
tent every day by all photographers
who employ color screens In their
work. and il is well known that In
creased contrast can often be obtained
In this way.
IT. for example. we attempt To pho.
tograph white clouds against blue
sky without the use Of such screen
we get scarcely any trace ~ the
clouds at all. whereas II we place
yellow glass In front OF the lens the
clouds come out snow white on very
dark background
We can see this same effect by view
Ing the clouds first through piece ~
deep blue glass. which Il what the
Photographic plate sees" and then
through the yellow plate.
Al my laboratory at East Hampton.
y. ". have pushed this principle 60
the extreme and have obtained photo
graphs both by the infra red and the
ultra violet invisible rays Robert VII
liam Wood IN Century