X-RAY MICROSCOPY 





PROJECTION GEOMETRY 



J- — t Source of Ultrcsoft 

 \ \ X-Radiation 



PHOTOMICROGRAPH 

 OF ERYTHROCYTE 

 (RANA PIPIENS) 



I 

 d 



Sample 



Film 

 20 Microns 



PHOTOMICROGRAPH 



OF CONTACT 

 MICRORADIOGRAM 



0.5x1^ SLIT-MICROPHOTOMETER 



TRACING OF 



CONTACT MICRORADIOGRAM 



Fig. 1. Illustrating the point of projection geometry utilized in x-ray microscopy. Interpretation of 

 the structure of the biological cell from the light microscope micrograph is made difficult because of 

 such effects as refraction and a very short depth of field. Contrast in the x-ray microscope picture is 

 simply and directly related to the chemistry and mass-thickness of the cell. 



tion since it measures the mass and ele- 

 mentary chemistry of the sample only, and 

 it is not sensitive to the effects of molecular 

 combination of the elements as is light ab- 

 sorption analysis. It is also of importance to 

 note that a wide range of x-ray wavelengths 

 are available for microscopy, e.g., 1 to 100 A, 

 with an associated range of absorption co- 

 efficients which vary by a factor of 100,000. 

 A considerable amount of research has 

 been reported since the time of the discovery 

 of x-rays on contact microradiography with 

 radiation in the 1 to 10 A region. In this 

 kind of x-ray microscopy, conventional x-ray 

 sources are used and the samples are placed 

 in contact with the photographic emulsion. 

 In 1951 Cosslett and Nixon introduced pro- 

 jection microradiography in which the sam- 

 ples are placed very near a point source of 



x-rays of micron dimensions and generated 

 by precisely focusing a demagnified electron 

 image of an electron source onto the target 

 material using magnetic lenses (see paper by 

 Ong Sing Poen, this volume). For nearly all 

 of this work with the soft x-rays (1 to 10 A) 

 the relatively thick samples required for 

 contrast limits the useful magnification to 

 about 300 X. However, for dense materials 

 of the heavier elements, resolutions have 

 been obtained with these radiations which 

 have permitted magnifications of the order 

 of 1000 X. Quantitative x-ray microscopic 

 analysis for both the mass and the elemen- 

 tary chemistry of biological materials was 

 first introduced by Engstrom in 1946. He 

 was also one of the first to recognize the 

 feasibility of using the ultrasoft x-radiations 

 for high resolution microradiographic analy- 



676 



