The river press. [volume] (Fort Benton, Mont.) 1880-current, 1910-06-01
LIGHT AND THE EYES
We Do Not Always See Things
as They Really Are.
COL03S WE CANNOT DISCERN

trate to the Regions Beyond the Vic.
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 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 ou by a lamp.
To make this a little clearer we may
consider one of the oldest experiments
in optics. the passage Of a ray of light
through a prism.
When a beam Of sunlight passes
through a prism Of glass such as Is
often found on old fashioned chande-
liers 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 n
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 nes present
in the spectrum. If we perform the
experiment in a dark room and place
a photographic plate in such a 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 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 4
greater or less amount of this ultra
violet light.
The only difference between this III
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
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 ever 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 us they do merely from the cir
cumstance that in the process of cvO-
luton u eyes have developed IL sensi-
bility to a certain region Of the total
spectrum of the light which comes to
us from the sun.
If we have used an ordinary photo
graphic plate in u 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. 1t
is sensitive only to a limited region
Recently. however, means have been
found of rendering plates sensitive to
the entire visible spectrum and to n
certain extent to the mira red or the
extension beyond the red analogous to
the II 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 other
than the nes with which we wish to
operate. This is done to a limited en-
tent every day by all photographers
who employ color screens in their
work. and it is well known that in
creased contrast can often be obtained
in this way.
If. for example, we attempt to pho.
tosraph white clouds against a blue
sky without the use of such a screen
we get scarcely any trace Of the
clouds at all. whereas If we place Il
yellow glass in front of. the lens the
clouds come out snow white on a 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 ~'sees," and then
through the yellow plate.
At my laboratory at East Hampton,
N. Y. I have pushed this principle to
the ezreme and have obtained photo.
graphs both by the infra red and the
ultra violet Invisible rays.-Robert viii.
liam Wood in Century.