Luminous spaces are seen separated from
each other by dark bands.
Every two adjacent spaces are in opposite mechanical conditions. On
one side of the dark band we have strain, on the other side pressure,
the band marking the neutral axis between both. I now tighten the
vice, and you see colour; tighten still more, and the colours appear
as rich as those presented by crystals. Releasing the vice, the
colours suddenly vanish; tightening suddenly, they reappear. From the
colours of a soap-bubble Newton was able to infer the thickness of the
bubble, thus uniting by the bond of thought apparently incongruous
things. From the colours here presented to you, the magnitude of the
pressure employed might be inferred. Indeed, the late M. Wertheim, of
Paris, invented an instrument for the determination of strains and
pressures, by the colours of polarized light, which exceeded in
accuracy all previous instruments of the kind.
And now we have to push these considerations to a final illustration.
Polarized light may be turned to account in various ways as an
analyzer of molecular condition. It may, for instance, be applied to
reveal the condition of a solid body when it becomes sonorous.
Pages:
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173