MOTOR RESPONSES IN INVERTEBRATES 575 



in the production of the physiological state which obtained before the 

 exposure, i.e., in recovery (Folger, 64). 



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

 cessation of movement vary with the intensity of the hght used; as the 

 intensity increases, the latent period increases rapidly from about 

 1 sec. at 500+ meter-candles to a maximum of about 6 sec. at 1000 ± 

 meter-candles and then decreases gradually to about 0.75 sec. at 11,000 ± 

 meter-candles ; and the light energy required to induce cessation of move- 

 ment decreases from about 7000+ meter-candle sec. at 500+ meter- 

 candles to a maximum of about 24,000+ meter-candle sec. at 1500 + 

 meter-candles and then increases to about 30,000 + meter-candle sec. at 

 11,000 + meter-candles (Folger, 64). These figures are only rather crude 

 approximations for they were obtained by a method of calculation which 

 yields results with a large probable error and they have not been con- 

 firmed. They 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 Amoeba proteus in which the responses are much 

 more consistent than they were in those used by Folger. 



No explanation has been offered for the variation in the latent period, 

 with luminous intensity during the stimulation period; but it has been 

 suggested that the variation in the amount of light energy required to 

 induce cessation of movement is due, at least in part, to adaptation 

 (Mast, 153). For if the fight is rapidly increased and then held, stream- 

 ing soon begins again, i.e., the organism recovers from the effect of the 

 increase in light, in other words, becomes adapted (Mast, 139 ; Folger, 64). 

 This shows that the effect of rapid increase in light is eliminated while 

 the organism is continuously exposed to the light, and it indicates that 

 there are two opposing processes involved, i.e., that increase in fight 

 induces certain changes in the organism and that internal factors tend 

 continuously to oppose and to eliminate these changes. If this is true, 

 the more rapidly a given amount of light is received, the less time there 

 is for recovery, and consequently the greater will be the effect of a given 

 quantity of light. This probably accounts for the increase in the amount 

 of light energy required (with decrease in intensity), observed in weak 

 fight, but it does not account for the increase in the amount required 

 (with increase in intensity), observed in strong light. 



The quantity of light energy required to induce cessation of movement 

 depends upon the chemical composition of the surrounding medium. 

 Increase in HCl, for example, causes increase in the quantity required, 

 but increase in CO2 causes decrease in the quantity required. In solu- 

 tions of KCl, CaCl2, and MgCU, respectively, the quantity of energy 



