X-IRRADIATION STUDIES ON THE MAMMALIAN RETINA 557 



before 



200 



Fig. 14. Similar experiments as in Fig. 13, except the ERG is evoked by a weaker 

 light stimulus and cathode ray sweep speed is slower. The responses in the center were 

 obtained during irradiations of 300 and 200 rads per min, respectively. Irradiation 

 was 10 rads per min for the reduced ERG to the right. Calibration: 100 mv. Time: 

 10 msec equals one horizontal dash. 



seen immediately after the start of x-irradiation. When x-radiation was dis- 

 continued, the b-\vave returned to the control level within the interval be- 

 tween two test flashes or slightly later. The eflfect also was tested after various 

 total x-ray doses delivered to the eye. As long as the total dose was below 

 that needed to produce the sudden irreversible ERG decline, changes of the 

 same magnitude as at the start of the experiment were readily demonstrated 

 by terminating radiation and resuming it. 



Further tests sought to determine whether or not x-radiation by itself 

 provokes an ERG if suddenly presented. A 34-in.-long lead brick in front 

 of the 2,000 kvp machine served as a "shutter" and by its fall, at the 

 desired moment, opened the x-ray beam to the eye so that within less than 

 10 msec the whole eye was exposed once the beam had begun to strike it. 

 The animal, in a completely light-shielded enclosure, was dark-adapted. 

 A response similar to that seen after the application of a very weak flash of 

 light was recorded following the "on" of 2,000 kvp x-irradiation (Fig. 15). 

 The lowest dose rate at which a small response was still provoked was 10"^ 

 rads per 100 msec. 



The photoexcitatory effect of x-radiation is clearly different from the 

 effect with which we were mainly concerned and which is closely related to 



Fig. 15. Response to the sudden "on" of 2,000 kvp at the indicated rads per min. 

 The electrode was a needle inserted into the anterior chamber. Calibration: 100 mv. 

 Time: one dash corresponds to 10 msec. 



