SELIG HECHT 



237 



in nature, which do not obey the reciprocity rule (Schwarzschild, 

 1899). However, when a process does proceed according to this 

 dictum, it is highly probable that its basis is a simple photochemical 

 reaction. 



The usual procedure in investigations of this kind is to vary the 

 intensity and to determine the time required to produce a given ef- 

 fect. This course was adopted with Ciona, and the results bore out 

 the Bunsen-Roscoe expectation (Hecht, 1918-19, a). Such a tech- 

 nique is entirely out of the question with Mya. The exposure re- 

 quired is so short that the errors of measurement would be too great. 

 It is simpler to keep the exposure time under control, and measure 

 the minimum intensity necessary to elicit a response with different 

 exposures. Six exposures were chosen for experimentation. Their 

 durations are given in Table I, first column. 



TABLE I. 



Relation between Exposure Duration and Minimum Intensity Necessary to Elicit 



Response. 



Eight animals which had been thoroughly dark-adapted were used. 

 Taking a given exposure, I subjected each animal in turn to it, and 

 noted whether it responded or not. After each animal had been given 

 a rest of at least 15 minutes, it was again exposed, nearer or farther 

 from the Hght depending on the individual's previous response. In 

 this way the distance at which a response was eUcited was gradually 

 approximated to the distance at which no response could be eHcited. 

 I considered a determination as finished when the "no response" 

 distance was 1 cm. farther than the "response" distance. Frequently 



