The procedure for radiographing a 

 sample of fish requires prelLmnary arrang- 

 ing of the developing and fixing solutions 

 in open trays, or tanks, in a darkroom. The 

 cassette, or X-ray film holder, is then 

 loaded in the darkroom, exposing the film to 

 yellovf or red filtered light only. The 

 cassette is placed in position on the X-ray 

 machine so that it is centered beneath the 

 X-ray tube and covered with wax paper to 

 prevent any damage from moist specimens. 



The specimens to be radiographed are 

 arranged on the wax paper and identified 

 serially by placing a lead number near the 

 first and last fish. The film also may be 

 identified by including lead numbers repre- 

 senting the collection number. After expo- 

 sure, the cassette is returned to the dark- 

 room where the film is removed and developed. 



As a routine precaution, dental films 

 are placed in the area immediately surround- 

 ing the operator to detect possible stray 

 radiation during e:-rposures. The films are 

 developed and examined after 5 to 10 minutes 

 of accumulated e:q50sure. Resvilts of all 



tests thus far have been negative. 



Viewing the finished negative can best 

 be accomplished by projecting diffused ligiit 

 through the film in the direction of the 

 viewer. The difference in transparency of 

 the film, resulting from differences in 

 density of the various parts of the fish, 

 become intensified by the diffi^.sed light, 

 and, consequently, facilitate the process 

 of counting vertebrae. 



A suitable viewer for standard size 

 X-ray films (8" X 10" and lU" X 17") was 

 constructed of plyijood and four 20-v7att neon 

 fluorescent tubes. This consists of rectan- 

 gular box, 20" X 31" X 6^r", covered witii a 

 ground glass plate measuring l8" X 29". The 

 fluorescent tubes, which are fastened to the 

 inside bottom of tne box, provide uniform 

 illumination of suitable intensity (fig. 6). 

 The film is placed directly on top of the 

 glass plate for viex-ring. Wnen counting 

 vertebrae, it is advisable to cover the film 

 with a clear glass plate to prevent unneces- 

 sary snradging and scratching of the fiLm. 



Figure 6. — Lightbox for viewing X-ray films of menhaden vertebrae. 



