' SELIG HECHT 245 



Physically speaking, this energy requirement means that a definite 

 mass of sensitive substance is decomposed by the light. When the 

 energy is delivered rapidly, that is when a high illumination is attained 

 suddenly, the necessary photosensitive substance is decomposed be- 

 fore the regeneration reaction sets in. But if the energy is dehvered 

 slowly, sufficient sensitive substance to produce a response cannot be 

 decomposed before the ''dark," regeneration reaction becomes effec- 

 tive. The "dark" reaction, once under way, proceeds slowly or 

 rapidly depending on the rate at which the light decomposes the 

 sensitive substance into its precursors. As a result, a stationary 

 state is attained analogous to a condition of true equilibrium, in 

 which no amount of exposure to Hght can form enough precursors to 

 initiate a response. Even if the intensity continues to increase to a 

 very high value^ no effect can be produced as long as the increase is 

 slow, simply because a new stationary state of the opposing reactions 

 will be reached if enough time is allowed. 



The time interval during which the necessary amount of energy 

 must be delivered is known experimentally. In Ciona the 5,000 units 

 of energy must be received within about 10 seconds; in My a the 

 required five units must enter the sense organ in less than 1 second. 

 The time hmit is thus greater in Ciona than in My a. This time inter- 

 val, as we have said, depends on the speed with which the regenera- 

 tion reaction proceeds. Therefore the regeneration reaction in Ciona 

 should be slower than in Mya. This is indeed the situation. Dark 

 adaptation is wholly dependent on the regeneration reaction (Hecht, 

 1918-19, a, h). In Ciona, dark adaptation requires about 4 hours, 

 whereas in Mya the same process is complete in about 35 minutes. 

 The time limit for the energy dehvery is thus proportional to 

 the velocity of the "dark" reaction which regenerates the sensitive 

 substance. 



The conclusion is clear. In spite of the apparent sensitivity of 

 animals like Mya and Ciona to sudden illumination the effective agent 

 for this sensitivity is a definite quantity of energy which obeys the 

 Reciprocity Law of photochemistry. The fact that this amount of 

 light energy must be received by the sense organs in a limited time is 

 merely a concomitant of the additional circumstance that the photo- 

 chemical reaction is reversible. The rate of change of the light inten- 



