| http://www.w3.org/ns/prov#value | - In the accompanying drawings, forming a part hereof, there is illustrated two forms of embodiment of the invention, `in which drawings similar reference char-acters designate corresponding parts, and in which: FIGURE l is an isometric view showing the raster, external illumination means, and two controlling circuits (shown in block) which apply voltages to the raster electrodes. v F
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