Development time at specific tempera- 

 tures are included for newly prepared solu- 

 tions, however, prolonged exposure to air 

 reduces the speed of the solutions so that 

 a longer period of development is required 

 after each use. 



Submission of the fiLm in the fixer 

 solution for 10 minutes usually is suffi- 

 cient to arrest all chemical action caused 

 by the radiation and developer and harden 

 the protective film emulsion. A longer 

 fixing time is required as the solution 

 becomes oxidized. After the fixing process 

 has been completed, the film should be 

 immersed in a flowing water bath for at 

 least an hour to remove all traces of chem- 

 icals before being dried and examined. 



The strength of the fixer can be pro- 

 longed by using a "short stop" bath of 28 

 percent acetic acid to rinse the film after 

 it is taken out of the developer solution} 

 the chemical strength of either solution 

 may be kept longer by storing in lightproof, 

 airtight glass or stainless steel containers 

 while not in use. 



X-RA7 PROCEDURE 



The procedure which we developed for 

 X-raying menhaden followed the usual pat- 

 tern of trial and error experimentation. 

 Initially, a number of test exposures was 

 made on a single array of specimens of 

 various sizes, altering the kilovoltage, or 

 expos\ire time, or both, until a satisfactory 

 negative was produced. This approach led 

 to considerable difficulty in interpretation 

 of results as three variables were Involved, 

 kilovoltage, exposure time, and size of 

 fish. It was noted, however, that consist- 

 ent results were obtained by either in- 

 creasing the kilovoltage and lowering the 

 exposure time, or lowering the kilovoltage 

 and increasing the time. This discovery 

 let to the consideration of only two vari- 

 ables, viz., kilovoltage and fish length 

 with time held constant. Interpretation of 

 the variability of the data thus was greatly 

 simplified, and radiography of acceptable 

 to excellent quality became routinely 

 possible. 



The relation of kilovoltage to fish 

 length is shown in figure U. The minimal, 

 optimal, and maximal curves were derived 

 from e:q50sures of several hundred menliaden 

 specimens of various lengths to all kilo- 



voltages which would produce a dark film 

 background at 10 ma. for 10 seconds at a 

 focal distance of 30 inches. Each film was 

 then graded for contrast between vertebrae 

 and surrounding tissue. All exposures 

 lacking sufficient contrast for clear reso- 

 lution of vertebral centri were considered 

 unacceptable, the remainder was graded as 

 acceptable or excellent. Examples of the 

 exposures of varying contrast are shown in 

 figure 5. 



The relative simplicity of fish radiog- 

 raphy is demonstrated in figure h by the 

 large tolerence limits between the maximum 

 and minimum kilovoltage s required for speci- 

 mens of particular* size and emphasize the 

 adaptability of the method for processing 

 random samples. For example, a kilovoltage 

 of 55 would suffice for X-raying a sample 

 of fish ranging in size from approximately 

 55 mm. to 180 mm., a size difference of 125 

 mm. To determine the appropriate kilovolt- 

 age for a sample, it is necessary to con- 

 sider only the smallest and largest speci- 

 mens in the lot; and since the smallest 

 fish are the most difficult to resolve, 

 exposures suited to these should be selected. 

 If size variation is great, it is suggested 

 that the sample be segregated into lots of 

 fish of similar size. Optimum exposures 

 for fish of various sizes may be determined 

 from the figure by inspection. All exposTires 

 were purposely made at 10 ma. for reasons of 

 thermal dissipation, othervise, a higher 

 milllamperage setting would have been justi- 

 fied, especially where large fish were 

 involved, as the time of exposure could be 

 reduced. The X-ray tube can be operated 

 continuously for several minutes at 10 ma. 

 without an excessive build-up of heat beyond 

 the thermal capacity of the tube. 



;o *□ 60 ao 100 



iBlI ISO zoo no 2«0 260 260 100 ilO 140 )«0 



i^igvire U' — Relation of kilovoltage to fish 

 size for standard X-r?.y film. 



