  The concept of nearsightedeness of electronic matter (NEM) was introduced by
W. Kohn in 1996 as the physical principal underlining Yang's electronic
structure alghoritm of divide and conquer. It describes the fact that, for
fixed chemical potential, local electronic properties at a point $r$, like the
density $n(r)$, depend significantly on the external potential $v$ only at
nearby points. Changes $\Delta v$ of that potential, {\it no matter how large},
beyond a distance $\textsf{R}$, have {\it limited} effects on local electronic
properties, which tend to zero as function of $\textsf{R}$. This remains true
even if the changes in the external potential completely surrounds the point
$r$. NEM can be quantitatively characterized by the nearsightedness range,
$\textsf{\textsf{R}}(r,\Delta n)$, defined as the smallest distance from $r$,
beyond which {\it any} change of the external potential produces a density
change, at $r$, smaller than a given $\Delta n$. The present paper gives a
detailed analysis of NEM for periodic metals and insulators in 1D and includes
sharp, explicit estimates of the nearsightedness range. Since NEM involves
arbitrary changes of the external potential, strong, even qualitative changes
can occur in the system, such as the discretization of energy bands or the
complete filling of the insulating gap of an insulator with continuum spectrum.
In spite of such drastic changes, we show that $\Delta v$ has only a limited
effect on the density, which can be quantified in terms of simple parameters of
the unperturbed system.
