Scintillating grid

Optical illusions, 5/5

\documentclass[tikz,border=10pt]{standalone}
\begin{document}
\begin{tikzpicture}
%% perception of black dots in the white circles
  \filldraw[black] (0,0) rectangle(12,9);
  \foreach \a in {1,...,7}
    \draw[line width=3mm,gray] (\a*1.5,0)--(\a*1.5,9);
  \foreach \b in {1,...,5}
    \draw[line width=3mm,gray] (0,\b*1.5)--(12,\b*1.5);
  \foreach \a in {1,...,7}
    \foreach \b in {1,...,5}
      \fill[white] (\a*1.5,\b*1.5) circle(0.2121);
\end{tikzpicture}
\end{document}