596 RADIATION BIOLOGY 



off-effect. Occasionally, as a result of very strong stimulation, there is 

 a silent interval followed by a short train of impulses after the hght goes 

 off. (6) The expected characteristics of single-nerve-fiber responses are 

 observed: the magnitude and form of the spike invariant with the dura- 

 tion, the intensity, the wave length, or other characteristics of the 

 stimulating Hght. 



Reciprocity of Time and Intensity of Light. Stimulus duration and 

 intensity are reciprocally related over wide ranges in the arousal of 

 impulses in the Limulus optic nerve fiber (Hartline, 1934). Evidently 

 the photochemical reaction within the sense cell is so simple and so accu- 

 rately represented by the nerve discharge that the Bunsen-Roscoe law 

 (/ X / = /v) is obeyed. For very short flashes, stimulation occurs entirely 

 during the latent period and has therefore ceased before the nerve response 

 begins to occur. This shows that the latent period is not that of the 

 photochemical reaction but rather that of subsequent processes whose 

 velocity and total extent are precisely governed by the photochemical 

 one. The intensities that are capable of affecting the response of a single 

 ommatidium certainly cover a range of more than six log units. 



Spectral Sensitivity. Another property related to the photochemical 

 process is that of spectral sensitivity. Graham and Hartline (1935) 

 measured the energy of light, at each of several different spectral loca- 

 tions, which was required to elicit an impulse discharge. They assumed 

 that a given amount of energy must be absorbed by the receptor cell in 

 order for it to discharge an impulse. They were therefore able to com- 

 pute an absorption spectrum for the photosensitive substance. The 

 resulting data showed a maximal sensitivity for the region between 500 

 and 550 m^ and a curve whose form is in general agreement with that of 

 the absorption spectrum of visual purple. 



Dark Adaptation. The course of dark adaptation in the sensory cells 

 of Limulus is surprisingly similar to that of higher animals, including 

 man. This conclusion, first obtained (Hartline, 1928) by the use of the 

 electroretinogram, was substantiated in more detail by later work on the 

 single receptor unit (Hartline and McDonald, 1947). 



Dark adaptation may be measured by preexposing the eye to steady 

 illumination and then determining the intensity of a test flash that is 

 just sufficient to evoke a single impulse in the optic nerve fiber. This 

 procedure, analogous to that of determining threshold intensities for the 

 human eye, shows that the critical intensity falls rapidly during the first few 

 seconds after the preadapting light is turned off. The critical intensity 

 then continues to decline at a progressively slower rate for an hour or 

 more under typical conditions. 



The dark-adaptation process takes longer after a strong preadapting 

 light than after a weak one. The intensity and duration of preadapting 

 exposure are nearly reciprocal quantities over a wide range. However, 



