A surveying optical diffractometer.

I N T R O D U C T I O N Optical diffraction and filtering of electron micrographs offers an objective method for analysing periodic structures. Because the preservation of detail in electron micrographs usually is not perfect, many micrographs must be examined and assessed to identify images suitable for analysis. I t is useful to have an instrument that allows one to survey many dozens of electron micrographs rapidly. This article describes such a surveying optical diffractometer which is compact, easily constructed, and produces good quality optical diffraction patterns. The major features of the surveying optical diffractometer evolved from experience in the use of an instrument described previously by DeRosier & Klug (1972). The features in common with the earlier instrument are (a) an intense light source (laser), which provides plane parallel light over a large area (160 mmn) of the specimen; (b) a folded optical path which permits all instrument controls to be placed within easy reach of the operator; (c) a movable carrier stage for the electron micrograph plate; (d) an adjustable specimen mask or slit system; (e) an optical projection system that permits rapid switching between an image of the specimen and its diffraction pattern; and (f) a 35-mm camera for recording either the specimen or its transform. The importance of these features is that since the diffraction pattern is bright, the quality of the optical transforms from different micrographs or from different regions within a micrograph can be very rapidly determined visually with this instrument. It is mounted against a wall and operated from a sitting position, much like a conventional light or electron microscope. The new features of this instrument, compared to that of DeRosier & Klug, are: (a) It is more compact, making the controls even more convenient to reach. (b) It is built from parts most of which are easily obtained commercially. (c) It does not have to be operated in a photographically dark room. (d) The double-mirror arrangement in the optical system produces a right-side-up image so that the motion of the image during surveying parallels motion of the micrograph. In the De Rosier & Klug machine, the single mirror produces an image that is upside-down and backwards, and the motion of the image is opposite that of the micrograph. (e) The addition of a diffraction lens improves the quality of diffraction patterns without any loss in ability to change the size of the diffraction pattern. The somewhat lower quality of * Present address: Department of Zoology, University of North Carolina, Chapel Hill, NC 27514, U.S.A.