XVII. 



RADIOACTIVE TRACERS 



037 



graphic emulsion will also differ, and the longer this path, the more 

 diffuse will be the spot on the photograph corresponding to a point of 

 location of a radioactive element in the original. For this reason or 

 particles with their short ranges and high energies give the best 

 radioautographs {3, p. 130). If a emitters were biologically impor- 

 tant they would find wide use for this purpose; since they aren't, it is 

 necessary to use the /S-ray emitters that are important and to be satis- 

 fied with more diffuse autographs. An element giving only X or 



Fig. 12. Radioautograph of zinc in a single tomato. This technique makes 

 visible 3 X 10"^ g. zinc present in each tomato seed. (Courtesy P. R. Stout.) 



7 radiation, or a very high proportion of such would be useless from a 

 radioautographic point of view. 



Also with a view to obtaining the least possible diffuseness and 

 the highest possible resolution, one must use tissue slices that are as 

 thin as possible. Best success has been obtained with a thickness of 

 5 n, though 10 M has often been used. Contact between the tissue 

 section and the photographic emulsion should be as intimate as it 

 can be made. If direct contact cannot be made, intervening material 

 should be thin; a film of celloidin or similar material has l)een used. 



