EFFECTS OF IONIZING RADIATION ON VISION 539 



of cells, the changes of the adaptation of the retinal and neural tissues, 

 the movements of the eye. and the many subtle changes in the state of the 

 organism. Recent electron microscope studies ha\e shocked visual research- 

 ers by showing that, in addition, the submicroscopic organization of the 

 photoreceptor can and does change greatly during light adaptation i Fernan- 

 dez-Moran. 1960 i. 



In summary, the visual system is an example of how the response of a 

 biologic system to a stimulus is determined by the organization of that 

 system, and this was shown for all structural levels of organization. Light 

 energy absorbed in the visual system triggers the release of greater amounts 

 of energy, and the latter the release of still more, etc., in a sequence of 

 energy-amplifying processes. High energy radiations can also trigger such 

 processes. The great sensitivity of a biologic system, such as the visual system, 

 is achieved by the great specificity of it organization, and, as a consequence, 

 small changes in the organization can produce large changes in the sensi- 

 tivity. Studies have shown such changes in the visual system produced by 

 comparatively small doses of high energy radiations. 



In conclusion, the author suggests, in \ iew of the principles e\idenced in 

 the abo\e discussion, that radiobiology be defined as the study of the 

 organization : at the molecular, submicroscopic, cellular, tissue, organ, and 

 organismic le\els) of the ensemble of atoms in part of which the radiation 

 energy is absorbed and in which the response is observed. 



Appendix: Effects of High Energy Radiation on Visual Function 



The known stimulating and damaging effects of high energy radiation on 

 \isual function are listed in Tables I and II. respectively. Most of the studies 

 listed have been discussed in earlier re\ lews ( Lipetz, 1955b, 1960a), so only 

 the newly added studies will be discussed here. 



Baylor and Smith i 1958) found that a small crustacean. Daphnia maona, 

 showed positive geotaxis i swimming downward > and negative phototaxis 

 (swimming away from a red light source i when exposed to sufficiently 

 intense x-rays. The threshold dose rate was between 160 and 180 r per min. 

 This behavior is typical of light stimulation of the violet receptor of its 

 nauplial eye. The heat and fluorescence produced by x-rays were insufficient 

 to account for this effect. The effect could be duplicated with reducing com- 

 pounds added to the water bathing the animals. 



Dawson and Smith i I959i recorded action potentials from a single optic 

 nerve fiber leading from the compound eye of the horseshoe crab, Limulus 

 polyphemus. It was found that as little as 5 r of x-rays caused a significant 

 decrease in the threshold intensity of a light flash capable of evoking such a 

 response in a fully dark-adapted eye. A cumulative lowering of the light 



