768 Wisconsin Academy of Sciences, Arts, and Letters. 
selective absorption, each pigment absorbs certain definite colors 
from white light and transmits or reflects only those which it does 
not absorb. A transparent object takes on the color of the 
light which it transmits, while an opaque object takes on that 
of the light which it reflects.* If an object absorbs all of the 
radiations of white light and transmits none, it is black; if it 
absorbs all but the red radiations, for example, it is red; but 
if it absorbs none and transmits all of the radiations of white 
light, it appears to be colorless. Again, if light of only one color 
be absorbed, violet for example, its complementary color, yel¬ 
low in this instance, will alone be visible. If on the other hand 
two complementary colors alone are absorbed, the object will 
appear relatively colorless. Many substances are quite trans¬ 
parent and colorless within the limits of the visible spectrum 
which show selective absorption in the infra-red or ultra-violet 
regions. Such substances only appear colorless. In a phy¬ 
sical sense they are similar to colored substances, for, physi¬ 
cally, there is little difference whether absorption is of radiant 
energy of short wave length in the ultra violet, of medium 
wave length in the visible portion of the spectrum, or of long 
wave length in the infra-red. 
That a substance shows selective absorption is probably due 
to the fact that the oscillation frequencies of its molecules cor- 
spond to certain definite wave lengths of radiant energy, 
the energy corresponding to such wave lengths being absorbed 
by the molecules. In accordance with this theory of color 
in material objects, much attention has been paid in recent years 
to the question of what molecular structures are likely to cor¬ 
respond to more or less definite periods of molecular vibra¬ 
tions, thus producing selective absorption. Narrowed down 
to the study of pigments, the question has taken the form of 
what particular configurations, as well as the introduction of 
what group or groups of elements, will throw this selective ab¬ 
sorption into the region of the visible spectrum where color 
will be produced. This brings the whole subject of chromo- 
phorous groups into a new light and explains how the intro¬ 
duction of a given group, supposed to aid in the production 
* Comparatively few substances, e. g. the metals, some solid organic dyes, feathers, 
etc., which have a metallic lustre, owe their color to reflection. Most substances gen¬ 
erally considered opaque transmit light for some distance below the surface. Color in 
such substances-is a transmission rather than reflection phenomenon. 
