X-KAY MICROSCOPY 



Fig. 5. Scheme of geometrical unsharpness 

 (from Engstrom) (54). S — specimen, F — film, A — 

 focal spot, b — distance specimen — focal spot, a — • 

 distance specimen — emulsion, U — penumbra. It is 

 evident that penumbra (U) is the greater the far- 

 ther the specimen is from emulsion. The closer it is, 

 the less its image is blurred hy divergent x-rays. 



tions, even if it seems insignificant at low 

 power, becomes more and more pronounced 

 with increasing magnification. This loss of 

 sharpness together with the "discontrasting" 

 effect of magnification diminishes evidently 

 the resolving power of the microradiograph. 

 No literature data are available on the 

 ratio between the thickness of specimen and 

 the maximum possible magnification of mi- 

 croradiograph. Cosslett (40) showed that 

 the resolution of microstructures in an elec- 

 tron micrograph of a section cannot be more 

 than one tenth of the thickness of the sec- 

 tion. In other words, one cannot expect to 

 see details smaller than one tenth of a 

 micron in an electron micrograph of a sec- 

 tion one micron thick. Although there are 

 no direct indications in the literature, ap- 

 parently a similar ratio exists in optical 

 microscopy. However, the optical resolution 

 of the microscope may be improved to some 

 extent by making use of the micrometer. 

 The use of this is very limited in the study 

 of contact microradiographs. The microm- 

 eter may be applied only for the better view- 

 ing of parts of microradiographical image 

 lying in different planes of the emulsion 



coat. Evidently, the above-mentioned ratio 

 has to be in microradiography other than 

 1 : 10 pointed out by Cosslett, probably 1 : 2 

 or even 1:1. According to our experience one 

 may expect to obtain a sharp x-ray image of 

 a microstructure, that is to say, an image 

 which remains sharp at maximum magnifi- 

 cation, if the radiographed section contains 

 merely this structure or is at least no more 

 than twice as thick as this structure. In 

 other words, it is possible from the geo- 

 metrical point of view to get a sharp image 

 of a structure 5 m in diameter only in micro- 

 radiographs of sections 5 ju or at the most 

 10 M thick. 



The contact between the emulsion and the 

 object is achieved by pressure on all parts 

 of the object. This may be done with a film 

 of some polyethylene resin (e.g. "Teflon," 

 "Alathon" of DuPont) 0.00001 in. thick (or 

 even thinner if available). These films only 

 slightly absorb x-rays, are resistant to alka- 

 lies and acids, and are structureless as seen 

 in microradiographs. 



In the cassette used by us (Fig. 6) the 

 plastic film is stretched on the interior (in- 

 ferior) side of the upper cassette cover. 

 When the cover is in place the plastic film 

 presses the object which is put on a fine- 

 grain plate in the middle of the cassette 

 base. After the plastic has worn out it may 

 be changed easily. In the middle of the cas- 

 sette cover there is a round opening 1 in. in 

 diameter over which a light-proof film is 

 stretched which does not absorb x-rays. 

 Aluminum is not good for this purpose be- 

 cause it is mostly transparent to light rays 

 in thin sections and has a structure con- 

 spicuous in microradiographs. Light-proof 

 films may be made in one's own laboratory 

 by bathing celloidine film (about 0.00001 in 

 thick) in an alcoholic solution (0.25 %) of 

 Sudan black. The entire upper part of the 

 cassette is covered with lead 2-3 mm thick 

 in order to protect the x-ray plate from any 

 secondary radiation emitted by air. The 



598 



