MOTOR RESPONSES IN INVERTEBRATES 609 



Hartline and Graham (80) demonstrated, however, that in single- 

 nerve elements the impulses are periodic in continuous illumination. 

 They demonstrated that immediately after the receptor is illuminated 

 the frequency of impulses increases, that this is followed by a relatively 

 long period without impulses, after which the frequency is much lower 

 but fairly regular, and that the frequency varies directly with the inten- 

 sity of the illumination. The absence of regular periodicity in the optic 

 nerve as a whole, is, therefore, doubtless due to fusion of the effect of 

 the action currents in the individual fibers composing it. There probably 

 are then, in the nerve elements, sensitive periods followed by refractory 

 periods; periods in which light induces, in the receptors, photochemical 

 changes in one direction followed by periods in which the reverse (restitu- 

 tion) occurs. But it is difficult to understand how a system which has 

 been altered by the action of light, can be restored without any change in 

 illumination. 



ADAPTATION TO LIGHT AND TO DARKNESS 



It has been observed by various investigators that the sensitivity 

 of insects to light depends upon the degree of adaptation. Dolley (53) 

 attempted to ascertain this relation quantitatively for Eristalis teyiax by 

 subjecting for different periods of time one eye to darkness and the other 

 to light of a given intensity, and then noting the degree of deflection from 

 the median diagonal in a field of light consisting of two equal beams 

 crossing at right angles. He concludes that as the time in darkness 

 increases, the sensitiveness increases rapidly to a maximum at about 

 60 min., then remains nearly constant for about 2 hr., after which it 

 decreases rapidly again, and that after the eye has been in darkness 

 4 hr. it is less sensitive than the light-adapted eye. 



These conclusions are, however, equivocal, owing to the fact that 

 in the test concerning the relative sensitiveness of the two eyes, after 

 unequal adaptation, the images in the eyes were not fixed, so that there 

 probably were marked changes in the intensity of the light received by 

 the illuminated rhabdomes. These experiments should therefore be 

 repeated and extended under more rigidly controlled conditions. 



Hartline (79) obviated this difficulty, but his observations were made 

 on the horseshoe crab, Limulus. He fixed the eye in relation to the source 

 of light and measured the action current produced by a given amount of 

 light on a given region of the retina after it had been in darkness for 

 various periods of time. He maintains that as the time in darkness 

 increases from zero, the action current increases rapidly at first and then 

 more slowly and that it becomes constant after about 30 min. in darkness. 

 If the action current, in millivolts, is plotted against the time in darkness, 

 the results lie with remarkable accuracy on the theoretical curve, 

 kt = [l/{Eoo — E)\ -\- C. He observed no decrease in sensitivity of the 



