SELIG HECHT 115 



A second source of error involves the change from light adaptation 

 to darloiess. It is again almost obvious that this change must be 

 clean cut and accurately timed. In almost all of the previously 

 published data this transition has involved the walking or running 

 of the subject from the light- adapting location to the dark room. It 

 follows that no measurements can be made during the variable interval 

 of running or walking. Even if such measurements were possible, they 

 would be of little significance for a study of the process, because 

 one could not tell when dark adaptation actually began. 



The third source of error involves the movements of the iris. At 

 least two distinct things happen when a light-adapted eye is removed 

 to darkness: the pupil dilates and the retina increases in sensitivity. 

 In order that the data may be made to mean something, it is necessary 

 to isolate the separate effects of these two factors. 



The means which I used to study foveal adaptation have taken 

 into account all these three sources of error. In addition I have 

 considered certain precautions to limit the measurements to the foveal 

 region. The entire procedure takes place in a large dark room. A 

 constant condition of light adaptation is secured by having the subject 

 look from a fixed distance at a brightly illuminated screen for a given 

 interval. The change from bright illumination to complete darkness 

 is made by shutting off the artificial source of illumination. The 

 result is a clean cut transition. Simultaneously with this change, 

 the subject is not even required to leave his seat. He merely raises 

 his head 5 cm. when the lights are turned off, and is at once ready for 

 measurement. 



In the preliminary experiments I attempted to take care of the 

 changes in the iris by the use of an artificial pupil. For several 

 reasons I abandoned this method. First, it was rather clumsy, and 

 proved to be a stumbling block in the accomplishment of a rapid 

 change from the position of light adaptation to that in which the 

 measurements were made. Second, the recent excellent experiments 

 of Reeves (1918) made it possible to correct for pupil variation. And 

 third, in consequence of the feasibility of this correction, it seemed 

 desirable to secure the gross results. This would enable one to make 

 comparisons with previous investigations, and would also give a 

 correct notion of the total change in the eye as a whole, which is 

 itself of some interest. 



