Clarke courier. [volume] (Berryville, Va.) 1869-1997, 1910-04-06
LIGHT AND THE EYES
We Do Not Always See Things
as They Really Are.
coL0RS 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
10 feel instinctively that we see things
as they are. This is a 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 4 lamp.
To make this a little clearer we may
consider one OF the oldest experiments
in optics, the passage of ray OF ligh,
through n prism.
When beam Of sunlight passes
through prism OF glass such rs l.
often found on old fashioned chanGc
liers the rays are spread ont. formin-
spectrum. which we see rs n rain
bow colored band upon the wall. The
colors OF the spectrum merge into ono
another 10y insenSible grada lions
though the older physicists were t.;
the habit OF recognizing seven-reG.
orange, yellow. green. blue. indi go anC
violet
These colors which the eye perceive
are by no means the only ones preseni
in the spectrum. II we perform the
experiment I'm a dark room and place
4 photographs plate in such n position
as TO receive the image Of the spec
trum we find that IL extends 10 n con
siderable distance beyond the violet.
In other words. there is 4 region be
yond the violet which the eye cannot
see. hut which the sensitive plate
able 10 photograph. This region T..
termed the ultra violet, and the light
from virtually all sources contains q
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
n glass prism for forming our spec
trum. we use one cut from n 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
clearcst 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 rs 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 sensi-
bllity 90 4 certain region OF the total
spectrum Of the light which comes to
Us from the sun.
II we have used an ordinary photo
graphic plate In our experiment we
shall find that Il 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 10 n limited region
Recently. however. means have been
found Of rendering plates sensitive t.
/ the entire visible spectrum and 10 n
certain extent 90 the mira rcd or the
extension beyond the red analogous "a
/ the one beyond the violet.
With a photogra phic plate rendered
sensitive % the entire spectrum we
could obtain an idea OF how things
would appear If our eyes were sensi
tive 10 regions other than the one
/ adopted 10y nature 10y excluding from
/ the lens OF the camera all rays other
/ than the ones with which we wish to
/ operate. This is done TO Il limited ex-
tent every day 10y all photographers
/ who employ color screens I'm their
work. and it is well known that in
creased contrast can often be obtained
in this way.
II. for example. we attempt TO pho
/ tograph white clouds against n blue
sky without the use Of such n screen
we get scarcely any trace OF The
/ clouds az all. whereas If we place n
/ 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 a piece Of
deep blue glass, which is what the
Photographic plate secs" and then
through the yellow plate
At my laboratory at East Hampton.
y. Y. have pushed this principle to
the extreme and have ob.~ined photo
graphs both by the mira ia ard the
ultra violet Invisible rays-Robert vii
liam Wood IN Century.