The Washington herald., 1915-12-26

example. appears red because 1t has the
ability to reflect chiefly red rays. Such 4
fabric will appear black when viewed un-
der a light which contains no red rays,
such as a blue or green light.
1t is a fundamental principle that except
in special cases a colored fabric cannot
appear the same under two different il
luminants. Therefore two colors that ap-
pear alike under one illuminant will not
match when viewed under another illum-
nant unless the colors in each case show
the same spectral character by analysis
In other words because the eye is no.
capable Of analyZing a color spectrally 1t
is possible to produce the colors which ap-
pear the same but whose spectral compo-
sitions differ. Such match will not re-
main a match generally under another IL
luminant differing in spectral character.
In this connection it is difficult to dis
tinguish a blue fabric as blue under ordi-
nary artificial light owing to the scarcity
of blue rays in most of the artificial illu-
minants. OF course pigment which was
absolutely true, that is monochromatic in
color. would not be changed in hue under
various illuminants but would be altered
in brightness. However, no monochro-
matte colors are found in practice but IL
pigments that were practically monochro-
matte existed very generally greater in
tensity of illumination would very often
he required than at present because such
>colors would reflect very little light.
striking instance of the effect of the
distribution of light is found in the case
OF the so-called changeable silks. Such
fabrics have a nap and when the fbers
end in the direction toward the light the
latter penetrates the fabric and is deeply
colored by multiple selective reflections.
The light that comes from other directions
is more or less specularly reflected, as II
from a mirror. thus undergoing less
change by selective absorption with the
result that various portions of the surface
appear differently.
The phenomenon OF after-images Is par
tcularly interesting. After the eye has
looked at a color 1t takes some time for
the sensation to cease. IL the filament
of an incandescent lamp be viewed for an
instant and then the eye be closed its
image will persist in the eye for some
time This has been called a positive
after-image. Soon depending upon the in
tensity of the. stimulus the image will
reach a stage of decay when II will appear
darker than the surroundings.
II the closed eyeld is illuminated the
visual field will appear brighter than in
the case where the eyelid is shielded from
the light by the hand being placed gently
against it. In the former case the after
image will remain positive shorter time
than in the case of the darker surround
ings. The same will be found when view
ing the afterimage against various white
or gray backgrounds with the eyelid open
Positive afterimages obtained by loo<-
ing steadily for a period at a white paper
illuminated by sunlight can be seen for a
brief period but they rapidly decay to a
brightness lower than that of ordinary
surroundings. In their decay they pass
through a series of hues. namely blue,
green. indigo. violet pink, dark orange
and so on which are more or less definite.
Afterinages are also produced by look
ing steadily at colored objects.
After-images play quite an important
part in vision. especially In viewing paint
ings and many other colored objects For
instance II pale blue sky line be viewed
in juxtaposition with a green landscape
the natural shifting Of the eye, even when
attempting moderately to gaze steadily at