dramatic increases in brightness could be achieved. What is most remarkable about this process, and a tribute to Phillips's genius, is that conventional photographic wisdom would dictate that this type of system should not work! Holographic plates initially contain a homogenous distribution of silver-bromide grains, and if all the developed silver were changed back into its original form, the plate would regain its virgin condition. In this condition, there would be no modulation of the incoming reconstructing light, since there would be nothing to differentiate where a bright fringe had been and where the dim ones had been. In reality, this process works really well, and the theory is that the developed grains migrate into the dim fringe areas as they are being rehalogenated. The bright fringe areas are represented by pure gelatin, and the dim fringe areas contain silver bromide, but the increase in modulable material in those areas increases efficiency. Since nothing leaves the emulsion-its elements are merely rearranged-the original thickness of the emulsion layer is preserved along with the spacing of the fringes during recording, so that it is possible not only to replay a reflection hologram with the laser that made it but to replace the hologram onto the object and generate real-time interferometric fringes with it. At first I doubted that this migration-diffusion mechanism would work, but its effectiveness was proven to me when I was making some extremely low frequency gratings. Ed Wesly (researcher, educator, artist), Center for Photonics Studies, Lake Forest College, 555 North Sheridan Road, Lake Forest, IL 60045, U.S.A., and the School of the Art Institute of Chicago, 280 South Columbus Drive, Chicago, IL 60603, U.S.A. Received 22 Febrtuary 1991. LEONARDO, Vol. 25, No. 5, pp. 439-442, 1992 439 This content downloaded from 207.46.13.169 on Mon, 04 Jul 2016 04:13:41 UTC All use subject to http://about.jstor.org/terms