lm; while
attached to the two parallel surfaces outside are two other pieces of
board, which represent the planes of vibration of the polarizer and
analyzer. On the two intersecting planes the waves are to be drawn,
showing the resolution of the first polarized beam into two others,
and then the subsequent reduction of the two systems of vibrations to
a common plane by the analyzer. Following out rigidly the interaction
of the two systems of waves, we are taught by such a model that all
the phenomena of colour obtained by the combination of the waves, when
the planes of vibration of the two Nicols are parallel, are displaced
by the _complementary_ phenomena, when the planes of vibration are
perpendicular to each other.)

In considering the next point, we will operate, for the sake of
simplicity, with monochromatic light--with red light, for example,
which is easily obtained pure by red glass. Supposing a certain
thickness of the gypsum produces a retardation of half a wave-length,
twice this thickness will produce a retardation of two half
wave-lengths, three times this thickness a retardation of three half
wave-lengths, and so on. Now, when the Nicols are parallel, the
retardation of half a wave-length, or of any _odd_ number of half
wave-lengths, produces extinction; at all thicknesses, on the other
hand, which correspond to a retardation of an _even_ number of half
wave-lengths, the two beams support each other, when they are brought
to a common plane by the analyzer. Supposing, then, that we take a
plate of a wedge form, which grows gradually thicker from edge to
back, we ought to expect, in red light, a series of recurrent bands of
light and darkness; the dark bands occurring at thicknesses which
produce retardations of one, three, five, etc., half wave-lengths,
while the bright bands occur between the dark ones. Experiment proves
the wedge-shaped film to show these bands. They are also beautifully
shown by a circular film, so worked as to be thinnest at the centre,
and gradually increasing in thickness from the centre outwards. A
splendid series of rings of light and darkness is thus produced.

When, instead of employing red light, we employ blue, the rings are
also seen: but as they occur at thinner portions of the film, they are
smaller than the rings obtained with the red light. The consequence of
employing white light may be now inferred; inasmuch as the red and the
blue fall in different places, we have _ir