X-RAY MICKOSCOPY 



The most perfect contact is easily achieved 

 in microradiography of sections of undecalci- 

 fied bone (see below); the most difhcult is 

 to get it in wet specimens. In the first case 

 the bone section has a smooth, dry surface; 

 in the second it may stick to the emulsion 

 and the subsequent tearing off damages the 

 emulsion. In other cases the emulsion is 

 damaged by alkalis or acids present in wet 

 specimens. Different methods are at hand 

 to avoid this complication. Bohatirchuk 

 (23) (26) (125) proposed to mount specimens 

 on thin sheets of plastic (e.g., "Alathon"), 

 and then place the mounted specimen 

 plastic-down on the emulsion. Specimens 

 mounted on plastic are prevented from 

 shrinking ; they may undergo coloring proce- 

 dures and may be preserved for a long time. 

 Engstrom (54) proposed to cover the emul- 

 sion with collodion, usually a few microns 

 thick, prior to mounting the specimen. After 

 radiography the collodion film is taken off 

 easily and the plate developed afterward. 



In some cases wet specimens may be dried 

 by putting them between two pieces of fine 

 blotting (or filter) paper which in turn are 

 pressed between two glass slides. This sim- 

 ple method is very good, especially in han- 

 dling thick sections. 



It is quite obvious that the system x-ray 

 tube: specimen: emulsion has to be held 

 motionless during microradiography. The 

 shrinking and the resulting motion of the wet 

 specimen during long exposures, especially 



inider vacuum, may be avoided only through 

 the shortening of the exposure time. Vibra- 

 tion of the building due to traffic or trembling 

 of the tube due to boiling water in the cooling 

 hoses are among the causes of unsharpness. 

 They should be foreseen and avoided. 



It is possible in some objects, e.g., bone 

 and teeth, to cut sections serially in planes 

 perpendicular one to another. This allows 

 the reconstruction of micro-anatomical in- 

 terrelations from their microradiographs. 



It is necessary to mention here that the 

 thickness (discussed in this paragraph) is 

 considered to be ecjual in all parts of the ob- 

 ject. This condition presents difficulties in 

 microradiography of natural (not sectioned) 

 objects. Evidently, a sharp microradio- 

 graphical image may be obtained only of 

 those parts of natural objects which are in 

 close contact with the emulsion and are thin 

 enough to permit the magnification of their 



miage. 



Fig. 8. Replica electron micrograph of unex- 

 posed and processed grains of a NTB emulsion (re- 

 produced from Boyd) (35). A. before, B. after de- 

 velopment (explanation in text). 



Graininess of the Emulsion. The mosaic 

 character of x-ray image is due to the silver 

 grains of the emulsion. The graininess is 

 revealed even at comparatively low magnifi- 

 cation in the usual emulsions of high sensi- 

 tivity. The emulsion for microradiography 

 must have the finest grain possible. Many 

 firms and companies in Europe and America 

 nowadays produce such emulsions. Only the 

 properties of the most frequently used Ko- 

 dak and Gevaert emulsions will be discussed 

 here. Eastman Kodak (USA) manufactures 

 fine-grain spectroscopic plates 649-0 or GH 

 and high resolution plates as well as other 

 samples. Gevaert (Belgium) issues Lipp- 

 mann films and Scientia 5e 56 plates. Emul- 

 sions of these samples of both companies 

 have very much in common. 



Regular size of grains of a fine-grain 

 emulsion is within the limits of a few milli- 

 microns (5-15 m/i). Grains acquire irregular 

 shape after processing and their size in- 

 creases (Fig. 8, reproduced from Boyd) (35). 

 Even distribution of grains and dense con- 

 centration are prerequisites of a sharp image 



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