432 Covtparative Animal Physiology 



used in obtaining that of Figure 13B. The four vertical rows of oscillograms 

 were made from the same dark-adapted liber with different durations of sin- 

 gle flashes of light. The four horizontal rows were taken at different intensi- 

 ties. It is to be noted that at unit intensity (horizontal row 1), as the dura- 

 tion of the flash is increased, the number of resultant impulses in the fiber 

 increases. Also, as the intensity is decreased (horizontal rows 2, 3 and 4), 

 the duration required to produce a single impulse is increased. The data indi- 

 cate that the reciprocity law (duration X time = constant) holds for the 

 production of a single impulse. 



RELATION BETWEEN ILLUMINATING INTENSITY AND FREQUENCY OF NERVE 



IMPULSE DISCHARGE. The discharge patterns of the optic nerve of Limulus and 

 of fibers A and B of the frog eye exhibit an "on" burst of high frequency. 



DURATION 

 0.0001 0.001 G.OI 0.1 



JIllllilllllilM lllilllilllllllllllilliillllll I >- 



z 



u 



ssacrs^gF °°^ 



0.001 



Fig. 136. Spike potentials recorded from single nerve fibers in response to illumination 

 of the eye with different intensities for various periods of time. Duration of exposure to 

 light is indicated at the top of each column in seconds; intensity in relative units is indi- 

 cated at the right (unit intensity 3,000,000 meter candles). The absence of the white 

 line indicates exposure of the eye. For very short exposures the arrows mark the position 

 of the signal. Time in 1/5 sec. From Hartline.*^ 



The relation of this maximal frequency to the intensity of the illumination 

 is interesting (Fig. 137), because there is a rough parallelism between this 

 relation and the relation of the retinal action potential magnitude to the 

 illuminating intensity (see Figs. 129 and 130, page 425). A similar relation 

 seems to hold between other parts of the optic nerve discharge pattern and 

 the illuminating intensity. 



DARK ADAPTATION. Employing the Linmhis single optic nerve fiber prepara- 

 tion, Hartline and McDonald'- investigated the dark adaptation of single 

 visual receptors. The recovery of a visual receptor after a short period of 

 adaptation to illumination of a moderately high intensity is shown in Figure 

 138. The nerve spike potentials are elicited by a short flash of light of fixed 

 intensity and duration. The records show that the number of spike 



