MOTOR RESPONSES 275 



After an amoeba has responded to rapid increase in illumination, 

 some time must elapse before it will respond again to the same increase 

 in illumination. There is therefore a refractory period, a period during 

 which the amoeba recovers from the effect of the stimulation. During 

 a part of this period the amoeba may remain either in light of the same 

 intensity, such as that which induced the response, in light of lower 

 intensity, or in darkness; but during the remainder of the period it must 

 be in light of lower intensity or in darkness. There are therefore two 

 processes which occur during the refractory period, one ( 1 to 2 minutes) 

 which proceeds with or without any change in luminous intensity, and 

 another (10 to 20 seconds) which proceeds only if the intensity is de- 

 creased. These processes result in the production of the physiological state 

 which existed before the exposure; that is, in recovery (Folger, 1925). 



The latent period and the amount of light energy required to induce 

 cessation of movement vary with the intensity of the light used (Fig. 94) . 

 Figure 94 shows that as the intensity increases, the latent period increases 

 rapidly from about one second at 500 ± meter-candles to a maximum 

 of about 6 seconds at 1,000 rh meter-candles, and then decreases gradu- 

 ally to about 0.75 seconds at 11,000 d= meter-candles; and that the light 

 energy required to induce cessation of movement decreases from about 

 7,000 Ht meter-candle seconds at 500 ± meter-candles to a maximum of 

 about 24,000 ± meter-candle seconds at 1,500 dz meter-candles, and 

 then increases to about 30,000 ± meter-candle seconds at 11,000 ± 

 meter-candles. These results are, however, only rather crude approxima- 

 tions. They were obtained by a method of calculation which yields results 

 with a large probable error and they have not been confirmed. The data 

 are, however, sufficiently accurate to substantiate Folger' s conclusion 

 that the Bunsen-Roscoe law does not hold. 



This work should be repeated, and the latent period established by 

 direct observation in all luminous intensities, instead of by calculation. 

 This is especially desirable since recent experience makes it possible to 

 select specimens of A. pro feus in which the responses are much more 

 consistent than they were in those used by Folger. 



No explanation has been offered for the mode of variation in the 

 latent period, with variations in luminous intensity during the period of 

 stimulation. However, it has been suggested that the variation in the 



