MEDICO-BIOLOGIC RESEARCH 



at high magnification. Unfortunately, these 

 properties cannot be controlled easily and 

 they vary therefore from one sample of 

 emulsion to another, and even within the 

 sample itself. There was an attempt to in- 

 crease the density of grains in some experi- 

 mental fine-grain emulsions (Blackett) (19). 

 Although the microradiographs of this au- 

 thor made on this emulsion appear to be a 

 step forward, there is no indication in the 

 literature as yet that the plates coated with 

 this emulsion are produced commercially. 



Thickness of the emulsion coat varies in 

 different emulsions, being in finest samples 

 around 5-10 /x. Sensitivity of fine-grain 

 emulsions is very low; if the sensitivity of 

 an ordinary x-ray emulsion is taken for one, 

 the sensitivity of Gevaert Lippmann emul- 

 sion will be 1/10,000 and that of No. 649-0 

 or GH spectroscopic plates of Eastman 

 Kodak about 1/15,000. Exposure time is 

 therefore quite long, from 2 to 60 min- 

 utes, depending in inverse ratio on the kilo- 

 voltage used and on the thickness of the 

 specimen. On the average, one tenth of a sec- 

 ond exposure on x-ray film will require about 

 16 minutes on Lippmann film and about 

 25 minutes on Kodak 649 (see also below). 

 Gevaert Lippmann films are more sensitive 

 at lower kilo voltages than Kodak spectro- 

 scopic plates. "Scientia" 5e 56 plates issued 

 by Gevaert not so long ago have very fine 

 grain and approximately the same sensitivity 

 as Kodak 649-GH samples. 



Resolving power varies in different sam- 

 ples. Producers of fine-grain emulsions claim 

 that the finest samples resolve about 1,000 

 lines per mm. This means that this finest 

 emulsion may resolve structures of 1 // size. 

 However, what is right for an almost two- 

 dimensional ruler-line is not always so in 

 relation to a biological three-dimensional 

 microstructure. A line is seen relatively sharp 

 even in the presence of grains, but not the 

 biological structure. Besides that, the resolu- 

 tion of objects 1 M in size comes across the 



limitation of geometrical resolving power 

 mentioned above. 



Processing of exposed fine-grain films and 

 plates does not differ much from that of or- 

 dinary material but it requires meticulous 

 cleanliness. All the solutions have to be fil- 

 tered every day, the glass ware thoroughly 

 cleaned. Washing has to be carried out with 

 distilled water, the drying to proceed in ex- 

 siccators. Ready microradiographs have to 

 be covered with cover glass for better pres- 

 ervation. In general, darkroom accessories 

 must be as in a chemical laboratory where 

 ([ualitative and quantitative analyses are 

 performed. D-19 or usual x-ray developer 

 produce good results (the latter 50% di- 

 luted). Time of development with these 

 developers is from 3 to 5 minutes. No ad- 

 vantage was found with other developers 

 recommended for fine-grain emulsions, in 

 particular microdot. The time of develop- 

 ment with microdol is much longer (10 to 12 

 minutes) and the achieved range of contrasts 

 is rather poor. 



A recipe for the preparation of a fine-grain 

 emulsion in any laboratory is given in the 

 paper by Bohatirchuk (125). 



Each magnification of any photo-image 

 has a somewhat "discontrasting" effect. 

 This is due to the fact that silver grains, 

 which seem to be solidly packed at a lower 

 magnification, appear separated at a higher 

 one. It is obvious that the smaller the grains 

 of emulsion are and the closer they are 

 packed in the emulsion layer, the less is the 

 discontrasting effect. 



It is accepted that the minimum difference 

 in contrasts which may be observed in 

 macroradiographs is about 30%. In other 

 words, one cannot distinguish blackness 1.5 

 from 1.2; only 1.5 from 2.0 or from 1.0. This 

 percentage is about 15-20 % in microradiog- 

 raphy in spite of the discontrasting effect of 

 magnification. Cytoplasm and cell mem- 

 brane, cytoplasm and nucleus may be differ- 

 entiated though their difference in contrasts 

 is even less than 15-20%. This is shown by 



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