THIRD HARMONIC MICROSCOPE. Imaging biological samples often involves telling apart one wet thing from another. A relatively new way of gaining the needed contrast is to exploit the nonlinear optical features of the sample itself by using a process in which a high-power laser beam can, when it is brought to a tight focus in certain media, generate subsidiary light waves at twice the original frequency (second harmonic), three times the frequency (third harmonic), and so on. If the detector is sensitive to just the third harmonic radiation, say, then by scanning the laser focus across the face of the sample, an image can be built up with a spatial resolution as small as the focal size. Jeff Squier at UC San Diego (619-534-0290, jsquier@chem.ucsd.edu and his colleagues have used this scheme to produce the first 3-dimensional third-harmonic image of a living system. (Paper JTuA2, May 25, at the electro-optics and quantum electronics meeting in Baltimore; see figure at www.aip.org/png/html/thirdharm.html.)