April 15, 1921] 



SCIENCE 



349 



sense organ. It is necessary to determine 

 whether this action of the light on the sen- 

 sory process possesses the ordinarily well- 

 demonstrated characteristics of photochemical 

 reactions. Photosensitive chemical reactions 

 have been studied extensively, and certain 

 of their properties have been found to be 

 commonly distributed. One of these is that a 

 definite quantity of radiant energy is asso- 

 ciated with a definite photochemical effect. 

 This is the well known Bunsen-Roseoe law, 

 which states that to produce a given effect 

 the product of the intensity and the time of 

 exposure of the light is a constant. 



Tested by this standard, the action of light 

 in the sensory responses of Oiona and Mya is 

 photochemical in nature. With Ciona, in the 

 production of a response, the sensitization 

 period varies inversely with the intensity, 

 and their product is constant and equal to 

 4,746 meter-candle-seconds. The same is true 

 for Mya. To produce the minimum stimu- 

 lating effect the intensity must vary inversely 

 as the exposure, the product of the two being 

 in this case only 5.62 meter-candle-seconds. 



Another common prox)erty of photochemical 

 reactions is that they possess a low tempera- 

 ture coefficient. Whereas ordinary chemical 

 reactions are markedly accelerated by an 

 increase in temperature, photochemical re- 

 actions proceed at pretty much the same rate 

 over wide ranges of temperature. Experi- 

 ments show that the temperature coefficient 

 for the action of light on the sensory activity 

 of Mya is 1.06 for a rise of 10° C. This 

 value is so characteristic for endo-energetic 

 photochemical reactions that, combined with 

 the applicability of the Bunsen-Eoscoe law, 

 it can lead to but one conclusion. That is 

 that the initial effect of the light in photic 

 stimulation is a rather simple photochemical 

 phenomenon. These results further indicate 

 that in order to produce a photosensory effect 

 a definite amount of a photosensitive sub- 

 stance must be broken down by the light. 



The third point which was made with 

 regard to the sensory responses of these ani- 



mals is that the continued application of the 

 light fails to elicit any additional effect. 

 This has been tested with intense sunlight 

 and with artificial light of over 10,000 meter- 

 candles intensity. The result is always the 

 same. After the first retraction of the 

 siphons, the animal comes into sensory equili- 

 brium with the light. The siphons are slowly 

 extended, and the animal appears to act as if 

 there were no light present. 



This brings us to the fourth characteris- 

 tic of photic sensitivity — the one which has 

 served as the key to the whole situation. This 

 is the fact that when an animal has come into 

 sensory equilibrium with a bright light, it 

 may be made to recover its sensitivity to 

 light by being placed for some time in the 

 dark. The rate at which this recovery takes 

 place is of significance, and has been care- 

 fully investigated in the case of Mya. 



An animal is exposed to an intense light 

 for an hour. It is then suddenly darkened, 

 and at regular intervals its sensitivity is 

 determined by measuring the reaction time to 

 a light of low intensity. What one finds is 

 this. For about three minutes the animal is 

 still insensitive to the particular intensity 

 used. On the fourth minute its first response 

 appears. The reaction time when measured 

 at this time is nearly twice as long as usual. 

 Measured at regular intervals, the reaction 

 time is found to decrease continuously during 

 the next thirty-five minutes. At first this 

 decrease is rapid, then slow, until after thirty- 

 five minutes or so it ceases entirely, and the 

 reaction time is at its minimum for that 

 intensity. 



The course of dark adaptation is very 

 orderly. It is similar in the case of Ciona, 

 except that it is much slower, requiring about 

 three hours for completion. 



What is the significance of these regular 

 changes? Physically they mean that during 

 dark adaptation the quantity of light re- 

 quired for a response is much greater lian 

 normal, and that this quantity decreases at 

 first rapidly, then more slowly. The effect 

 of the light we have shown to be the photo- 

 chemical decomposition of a sensitive sub- 



