Speed of light is a variable

Citation http://www.rpi.edu/dept/phys/Dept2/APPhys1/optics/optics/node4.html
Cite DJ Wagner at Rensselaer Polytechnic Institute

The speed of light has a maximum velocity of 2.99792458 x 10^8 m/s in a vacuum.

The index of refraction of some common materials are given below.

material	n	material	n
Vacuum	1	Crown Glass	1.52
Air	1.0003	Salt	1.54
Water	1.33	Asphalt	1.635
Ethyl Alcohol	1.36	Heavy Flint Glass	1.65
Fused Quartz	1.4585	Diamond	2.42
Whale Oil	1.460	Lead	2.6

Values of n come from the CRC Handbook
of Chemistry and Physics

v = c/n
Where
v = velocity of light through the medium
c = maximum speed of light in a vacuum
n = optical density or refractive index of media


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v(lead) = c/n(lead)= (299792458 m/s) /(2.6) = 115304791.538 m/s through lead.

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Dark Energy has a refractive index of ~<1. Though quite small Dark Energy still affects the speed, wavelength, frequency, and temperature of the photon-wave over great distances.

This is very important. I am beginning to think that the only constant is change.