The Marion daily mirror. (Marion, Ohio) 1892-1912, 1910-01-25
WHAT THE NORMAL
EYE CANNOT SEE
Our Eyes not Sensitive to
Regions Beyond the Violet.
INTERESTING EXPERIMENTS
Photographs Recently Obtained with
Supcr-scnsitive Plates, Both by
the Infra-Rcd and the UItra-vio-
let Invisible Rays.
From Robert William Woods "a New
Departure in Photography in the
Midyinter Cont,rv

Departure i'm Photography" in the
Midwinter Century.
Those of us who are fortunate
enough TO posses normal eyes are apt
to feel instlnctively 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 a lamp.
To make this a little clearer, we
may consider one of the oldest exper-
iments 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-fashloned chandeliers, the rays
are spread out. forming a spectrum,
which we see as a rainbow-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, organe.
yellow, green, blue, indigo and violet.
These colors which the eye perceives
In the spectrum. If we perform the
experiment In dark room. and place
a Photographic plate in such posi-
ton as to receive the image of the
spectrum. we find that II extends to
considerable distance beyond the vio-
let. In other words. there Is a region
beyond the violet which the eye can
not see, but which the sensitive plate
Is able to photograph. This region
is termed the ultraviolet. and the
light from virtually all sources con-
tains greater or less amount of this
ultraviolet light.
The only difference between this
ultraviolet 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. If, instead of using
glass prism for forming onr spec
trum, we use one cut form quartz
crystal, we find that the ultraviolet
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 he as Opaque as sheets of iron,
and polished silver would resemble
anthracite coal. In other words,
things appear as they do merely
from the circumstance that, i'm the
process of revolution. our eyes have
developed sensibility to 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 our experiment, we
shall find that IL has received no inn
preston from the red and yellow part
of the spectrum and very little from
the green. The plate. therefore, suf-
fers from the same defect as our eye;
II is sensitive only to limited region.
Recently. however, means have been
found of rendering plates sensitive to
the entire visible spectrum, and to a
certain extent to the infra-red, ir the
beyond the red analogous to
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 sensl-
tive to regions other than the one
adopted by nature, by excluding
firm the lens of the camera all rays
other than the ones with which we
wish to operate. This is done to "A
limited extent every day by all pho
who employ color screens
IN their work. and II Is well known
hat increased contrast can often be
obtained In this way.
II, for example we attempt to pho
.ograph white clouds against blue
ky without the use of such screen,
we get scarcely any trace of the
,louds at all. whereas If we place q
fellow glass I'm front of the lens. the |
LIQUIDS come out snow white on a
,ery dark background
We can see this same effect by
,iewing the clouds first through a
,lece of deep blue glass, which is
What the Photographic plate Sees"
md then through the yellow plate.
Last summer, at my laboratory at
Cast Hampton, Long Island, l pushed
his principle to the extreme, and
rave obtained photographs both by
he lnfra-red and the ultraviolet in
lsible rays.