214 THREE-DIMENSIONAL PHOTOGRAPHY
Imagine one of the combs held with its teeth perpendicular and
spaced about an inch from it, another with its teeth horizontal. If
you pass the vertical tinsel ribbon through the teeth of the first
comb, which can be done easily, it will not pass the teeth of the
second comb because the edge of the ribbon lies across the teeth
of the second comb. However the second (horizontal) tinsel rib-
bon can be passed through the second comb but not through the
first.
The action of a polarizer is crudely analogous to that of the
comb upon the glued tinsel. It permits the passage of a ray of
light which is vibrating in only one major plane. If a second
polarizer is held before the ray in such a position that it would
polarize the ray at right angles to its first polarization, the ray of
light cannot pass.
We have the apparent paradox of two substantially transparent
bodies which freely transmit light in a certain position, but when
given another relationship they become opaque and will not trans-
mit light.
This cancellation of light by a second polarizer when light has
already been polarized, is widely used, Inasmuch as light is polar-
ized by reflection at some angle from any non-metallic surface,
much of the daylight about us is polarized. Thus the glare from
a paved street is largely polarized and the use of polarizing goggles
eliminates this glare without a corresponding reduction in the
normal light intensity. The same thing is done by the photog-
rapher who often eliminates undesirable reflections, as in a win-
dow, by using a polarizer over the camera lens.
Polarized light is used widely in the study of crystals and min-
erals under the microscope, but it is used with certain accessories
so that the polarization may be that just described and known as
“plane” polarization, or it may be circular or rotary polarization.
The light may be split into two rays which are differently re-
tarded by the specimen under examination. The two rays are
mixed, and in mixing they interfere, producing very beautiful
color effects which are of great value in the study mentioned. But
these effects are outside our province.
Polarized light may be reflected and if the reflection is from a
metallic surface, the polarization is retained, but if the surface is
non-metallic the polarization is lost. The fact is of the first im-