The Hattiesburg news., 1910-02-26
LIGHT AND THE EYES
We Do Not Always See Things
as They Really Are.
C0L0RS WE CANNOT DISCERN


Normal Vision is Not Able to Pene-
trate to the Regions Beyond the vio-
let-some Results That Have Been
Attained With the Camera.
Those Of us who are fortunate
enough to possess normal eyes are apt
to feel instinctively that we see things
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 a prism.
When beam of sunlight passes
through a prism Of glass such as is
often found on old fashioned chande-
Hers the rays are spread out, forming
a spectrum, which we see as a rain
bow colored band 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 perceives
are by no means the only ones present
In the spectrum. If we perform the
experiment in a dark room and place
a Photographic plate in such position
as to receive the image Of the spec.
trum we find that 1t extends to a con-
siderable distance beyond the violet.
In other words, there is 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 ul-
ira violet light and ordinary visible
light is in the length of the wave.
which is, in fact, tie only difference
, between the different colors of the
visible spectrum. If, instead Of using
a 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.
| If your eyes were sensitive to this
region Of the spectrum instead Of to
the other the appearance Of things
would appear black. our windows
would be as Opaque as 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 sensi
bility to a certain region of the tota
spectrum of the light which comes tC
us from the sun.
If we have used an ordinary photo
graphic plate in our experiment we
shall find that it 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 beer
found Of rendering plates sensitive tt
the entire visible spectrum and to Q
certain extent to the infra red or the
extension beyond the red analogous t.
! the one beyond the violet.
With a Photographic plate rendered
sensitive to the entire spectrum We
could obtain an idea of how things
would appear if our eyes were sensi
tive to regions other than the one
adopted by nature by excluding from
the lens Of the camera all rays othel
than the ones with which we wish t.
operate. This is done to limited ex
tent every day by all photographer.
who employ color screens In theil
work, and it is well known that in
creased contrast can often be obtained
in this way.
If. for example, we attempt to pho
tograph white clouds against a blu
sky without the use Of such screel
we get scarcely any trace of tn,
clouds at all. whereas If we place
yellow glass in front of the lens tn
clouds come out snow white on YER
dark background.
We can see this same effect by view
ing the clouds first through piece O
deep blue glass. which is what tn
Photographic plate "sees." and the
through the yellow plate.
| At nv laboratory ot Fact napptor
N. Y, have pushed this principle t
the extreme and have obtained photo
graphs both by the ii1fra red and tn
ultra violet Invisible rays.-Robert Wi
liam Wood in Century