X-RAY MICROSCOPY 



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2. J. Opt. Soc. Amer., 38, 7G6 (September, 1948); 

 also a manual prepared by A. V. Baez. 



G. L. Clark 



Fig. 3. Enlarged images of 350-mesh screen in 

 x-ray microscope. (Kirkpatrick.) Upper left, full 

 image from two mirrors; upper right, partial image 

 from horizontal mirror; lower left, partial image 

 from vertical mirror; lower right, direct radiation. 



left, and the large spot, lower right, caused 

 by direct radiation). 



While this development is still in its early 

 stages both in theory and in practice, it is 

 evident that there is every prospect of a 

 successful x-ray microscope. Elliptical sur- 

 faces already have been found to be superior 

 to spherical or cylindrical ones in tests with 

 mirrors made by coating a spherical mirror 

 with a continuously variable amount of gold. 

 Magnifications of 70 diameters already ob- 

 tained are bound to be exceeded. The optical 

 system is also suitable for focusing of very 

 soft x-rays Avith wavelengths up to 45 A used 

 for diffraction analysis of structures of 

 materials with very large d spacings and 

 microradiography of single cells (cf. article 

 on Ultrasoft X-ray Microscopy, by B. 

 Henke) ; and for focusing of neutrons. As the 

 very newest development is the use of a sys- 

 tem of cross-reflecting mirrors to focus solar 

 x-rays in an X-Raij Telescope, as described 

 by A. V. Baez in the Encyclopedia of Spec- 

 troscopy (q.v.). 



REFERENCES 



1. Clark, G. L., "Applied X-rays," 4th ed., Mc- 

 Graw-Hill Book Company, N. Y., pp 105- 

 106, 1955. 



TWO-WAVE (BUERGER) MICROSCOPE 



In the final analysis of ultimate crystal 

 structures in terms of the motif, or complete 

 configuration of a molecule serving as a point 

 which is translated according to a definite 

 repeating plan, a Fourier series is summed 

 up. This is the same process as the super- 

 position of many sets of interference fringes 

 in a microscopic image. Hence optics pro- 

 vides several methods of Fourier synthesis 

 in place of mathematical calculations. 



The ultimate extension of the optical- 

 analogue method is the two-wavelength mi- 

 croscope. Since x-rays cannot be focused as 

 conveniently as visible light (cf. article on 

 Reflection Microscopy (X-Rays)) it is pos- 

 sible only to collect a diffraction pattern. 

 But W. L. Bragg also conceived the idea 

 that, if visible light could be made to con- 

 tinue in the paths of the x-ray diffracted 

 beams, it could be focused to give an image 

 of the crystal. Such a two- wavelength 

 microscope (Fig. 1) has been successfully 

 constructed by Buerger at IMIT, starting 

 with a diffraction pattern photographed in 

 his precession camera (an undistorted recip- 

 rocal-lattice photograph) which is equivalent 

 to the interference pattern that would be 

 formed by visible light and photographed 

 by a lens. A replica of the x-ray pattern 

 is made by boring holes in a brass plate 

 and is placed in an optical system that 

 produces an interference pattern which is 

 the image of the original crystal. The most 

 complex part of the apparatus involves the 

 use of mica plates behind the holes in the 

 brass plate capable of being tilted to produce 

 phase shifts by varying the length of optical 

 path. Individual rows of holes produce opti- 

 cal patterns as lines at right angles to the 

 rows similar to the Bragg-Huggins fringes. 

 The total effect is optical simmiation of all 



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