Java applet written to draw the Harrison construction of free electron Fermi surfaces for a 2-D square lattice. After Figure 11, Chapter 9, "Introduction to Solid State Physics", 8th Ed., by Charles Kittel. See also Problem 5, Chapter 9, "Solid State Physics", by Neil W. Ashcroft and N. David Mermin. Use the "Color" choice box to select "Gray" when printing on a black & white printer, or choose "None" for faster display rendering. The left-hand slider controls how many electrons per unit cell. The right-hand slider controls how many Brillouin Zone boundaries are drawn. Set either slider to zero to hide the feature.