ULTRAVIOLET MICROSCOPY 



Conclusion 



In summary, then, we have produced a 

 Fresnel zone plate consisting of a set of gold 

 bands supported by radial struts, leaving 

 alternate bands almost completely empty, 

 thereby making possible the transmission 

 and focusing of soft x-rays and extreme 

 ultraviolet radiations. We have tested it in 

 visible light and in the ultraviolet only as 

 far as 2537 A. All tests indicate that the 

 experimental resolution and transmissivity 

 are what one would expect from simple 

 theoretical considerations. This means that 

 from the point of view of resolution and 

 exposure the zone plate is a great improve- 

 ment over a pinhole. It compares favorably 

 in resolution, but not in speed, with a simple 

 lens, if we make the comparison in the visible 

 region. Below 1000 A, however, the speed 

 of the zone plate would be thousands or 

 millions of times greater than that of a 

 lens made of any known material. All the 

 experimental evidence now in existence 

 seems to indicate that there is a region in 

 the euv where the zone plate could serve as 

 the only practical focusing device, because 

 the reflectivity and the transmissivity of 

 known materials would make the use of 

 lenses or mirrors at normal incidence either 

 difficult or impossible. 



The lines of future research now seem 

 clear. First, experiments to focus soft x-rays 

 and euv radiation between 10 A and 1000 A 

 should be made since in this region the con- 

 ventional methods of image formation seem 

 hkely to break down. Next, as the zone 

 plate is a highly chromatic device, sources 

 of essentially monochromatic radiation 

 must be found for it. Several schemes suggest 

 themselves at once. For example, limiting 

 stops placed strategically can absorb the 

 radiation of one wavelength while almost all 

 else proceeds essentially unaffected. If we 

 could precede the circular zone plate by 

 similarly seK-supporting diffraction grat- 

 ings, new types of euv spectrographs with 



little or no astigmatism might become 

 feasible. 



The uses to which these zone plates may 

 be put in the astronomical telescopes of the 

 future will be determined by the problems 

 that need to be explored in astrophysics. 

 Because of the great intensity available from 

 the sun some early solar experiments would 

 probably be in order. The isolated lines at 

 around 150 and 300 A in the spectrum of the 

 sun's corona (19, 20) seem ideal for an 

 experiment with a zone plate telescope 

 •■'tuned" to one of these fines. Next in in- 

 tensity — and in possible interest — might be 

 the moon and the planets. The present 

 limitations on speed indicate that we must 

 develop means of greatly increasing the 

 effective speed of a zone plate device before 

 it can be useful in gathering euv and x-rays 

 from the stars (18). 



REFERENCES 



1. Symposium on X-ray Microscopy and Micro- 



radiography, Cambridge, England, August 

 16-21, 1956. The proceedings of this confer- 

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 V. E. Cosslett, Arne Engstron and H. H. 

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 radiography." Academic Press, Inc., New 

 York, 1957. 



2. Second International Symposium on X-ray 



Microscopy and X-ray Microanalysis, Stock- 

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3. Jastrow, R., /. Geophijs. Res., 64, 1647 (1959). 



4. Roller, L. R., "Ultraviolet Radiation," 



Chap. 5, p. 146, John Wiley and Sons, Inc., 

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5. Whipple, F. L., and Davis, R. J., Astron. J., 



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6. Hass, G., and Tousey, R., J. Opt. Soc. Am., 



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7. Rieser, L. M., Jr., J. Opt. Soc. Am., 47, 987 



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8. CoMPTON, A. H., AND Allison, S. K., "X-rays 



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 IV, p. 305 et seq., D. Van Nostrand Co., 

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9. KiRKPATRICK, p., AND BaEZ, A. V., /. Opt. 



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10. KiRKPATRICK, P., AND PaTTEE, H. H., Jr., 



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