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Strutt, JW (Lord Rayleigh). **
On the
Maintenance of Vibrations by Forces of Double Frequency, and on the Propagation
of Waves Through a Medium Endowed with a Periodic Structure**. Phil. Mag.,
S.5, vol.24, no.147, August 1887, pp.145-59. *Please see the discussion that
begins on page 156 and the formula for the stop-band-width, equation (74).*

Strutt, JW (Lord Rayleigh). **
On the
Remarkable Phenomenon of Crystalline Reflexion described by Professor Stokes**.
Phil. Mag., S.5, vol.26, no.160, September 1888, pp. 256-65.

Darwin CG. **
The Theory of
X-ray Reflection, Part II**. Phil. Mag., vol.27, 1914, pp.675-690.*
Darwin applied Rayleigh's ideas to "Dynamical X-ray Diffraction", i.e. the
theory which includes the effect of X-ray standing waves, finite mini-gaps, etc.
X-ray index contrast is too small to allow for a 3-d photonic bandgap. *