The speed of light is dependent on the medium
it is travelling through. To find the velocity of light in any given substance, one must use
n = c / v
That is the index of refraction of the substance equals the speed of light in a vacuum divided by the speed of light in the substance. Since, by definition, n cannot be smaller than 1, the velocity of light is the fastest in a vacuum. All other mediums slow light down.
Thus, the speed of light is determined by dividing the speed of light in a vacuum by the index of refraction of the medium.