Why does refractivity not depend on density; Lorentz-Lorenz

[Soren]

I must be stupid but my textbook states that the refractive index of a media does not depend on the density according to the Lorentz-Lorenz formula (LL)
$$(\frac{n^2-1}{n^2+2}) = \frac{4\pi Ne^2}{ 3m} \frac{1}{\omega_0^2 \omega^2}$$
Specifically it (Physical Optics, Akhmanov Nikitin pp.367) says: ".. since ρ ~ N, it follows from (LL formula) that the quantity
$$r =(\frac{n^2-1}{n^2+2}) \frac{1}{\rho}$$
should not depend on density"
Are they simply telling me that ρ appears on both sides of the equation?

 

[nasu]

They don't say that the index of refraction does not depend on density. At least not in the quote you give.
That "r" is not the index of refraction.

 

[Soren]

no you are right, r is called the specific refractivity of the substance. I suppose that is what they are trying to tell me, that specific refractivity (r) does not depend on density, whereas the refractive index of a substance does.